Thoughts on Australia

Much of the world's focus at the end of 2019 and the beginning of 2020 was on the massive heat waves and fires that afflicted the country of Australia. Multiple reports have suggested that over 1 billion animals have died in the blazes.  Did a change in climate play a role in this?  The answer is not a simple one — it never is — because there are other factors involved that should not be overlooked.  But there is sufficient evidence to implicate a warming, drying climate in the heat waves and fires that the island continent has been experiencing.

For starters, Australia suffered from a massive heatwave.  Obviously heat waves are more likely in the warming world. But what about the fires themselves? In a warming world, wet areas tend to get wetter and dry areas tend to get dryer. A regrettably prophetic 2016 study had suggested that California was becoming more vulnerable to forest fires due to a warmer and drier climate, well before the fires that afflicted the state over the last two years. And as it turns out, a paper concerning bushfires in southeastern Australia and their connection with climate change was published in 2007.  It stated, “A study conducted in 2005 examined the potential impacts of climate change on fire-weather at 17 sites in southeast Australia. It found that the number of ‘very high’ and ‘extreme’ fire danger days could increase by 4-25% by 2020 and 15-70% by 2050.”  So the potential for fires to get worse in Australia as the climate warms has been known and documented for at least a decade.

But other factors have exacerbated the extent and damage of this summer’s fires.  One factor is natural: a periodic oscillation called the Indian Ocean Dipole (IOD).  In the positive phase of the IOD, warm waters off the African coast lead to high rainfall in Africa, relatively cool and dry conditions in Indonesia, and relatively hot and dry air in Australia.  That is the present phase of the IOD, making it an important contributor to the fire season.  But there is also the factor of human carelessness and stupidity.  Much press has been devoted to the large number of fire-related arrests in Australia over the last couple of months, including over 40 deliberate fires and well over a hundred more accidental fires.  While that behavior is inexcusable, it is also not unusual; a 2009 survey by the Australian Government concluded that “between 2001–02 and 2006–07, the number of bushfires in Australia varied from approximately 46,000 to 62,000 per year, with an average of nearly 54,000 fires per year. This agrees quite closely with the average of nearly 52,000 fires per year calculated by the Australian Institute of Criminology using data from fire agencies from 1995–06 to 2005–06. It is estimated that 50 percent of fires are either deliberately lit or suspicious in origin.”  This leads to two important conclusions.  The first is that people directly affect the number of bushfires that get started, just like they played a role in the California wildfires.  The second is that a normal amount of human activity cannot begin to account for the extraordinary extent of the damage that this summer’s Australian fires have caused.

But if we are talking about the effects of a changing world on Australia, it is important to acknowledge how the politics and media of Australia are not helping the situation. This past year, the Australians held an election where climate change was very clearly on the agenda.  The party that wanted to do something about climate change lost to the party that didn’t. Why did this happen? Australian voters were more fearful of the consequences of doing something than they were the consequences of doing nothing. This happened despite the heat waves the country has recently experienced, and the Great Barrier Reef suffering massive bleaching events in the past couple of years to the point that it is in danger of dying altogether if the planet continues to warm. This fear of the cost of acting against climate change has been fueled in part by coal interests, predictably, but also the media -- especially Rupert Murdoch (the owner of News Corp. which runs Fox News in the USA), who owns 57% of the newspapers sold in Australia. (That statistic comes for a fairly old study, to be fair, but there's no reason to think that percentage has shrunk in the meantime.) That is a lot of control over a free press for one person to exert.

In 1998 the Australian rock band Midnight Oil, known for their advocacy of aboriginal land rights and their environmental activism, released a concept album called Redneck Wonderland about the state of Australia at the time. The album was prescient in many ways about how politics would change not only in Australia, but also in places like the United States and the United Kingdom, in the subsequent two decades. A song from that album called “Comfortable Place on the Couch” describes how people sit on the couch and absorb whatever their TV channel or newspaper (or website, if you bring it to the present) of choice tells them, without actually going out into the world and experiencing it for themselves. In the chorus, Peter Garrett sings “Some say the truth is what you see, I know the truth is what you feel” — predating Stephen Colbert’s more concise expression “truthiness” by a few years, and also predicting people’s stubborn indifference to what is going on right in front of their faces.  In many ways, this sums up what Australia has become today. Sydney is burning, the koalas are in danger of going extinct, the Great Barrier Reef is dying, and Australians reelected the people who want to sell a billion people in India all the coal that they can burn. As a climate scientist, I can only hope that the rest of the world doesn't wait till the change is right in front of them before they act to stop it.

(For updates on new posts, please click the "Follow" button.)

The Temperature in 2019

As is typical at the beginning of any new year, the scientific agencies responsible for global temperature records have all recently released their reports of the state of global temperatures in the year 2019.  With one exception, the agencies reported 2019 as the second warmest year on record. This is not really a surprising result; it has been clear from the middle of last year that the moderate El Niño event would make 2019 warmer than every year besides 2016 and possibly 2017.  There are a number of articles available that report on or summarize the overall findings, and I would particularly recommend the article that Zeke Hausfather published on the CarbonBrief site on Monday, but I would like to focus on one particular detail in the report that Dr. Jim Hansen published last week concerning the temperature data produced at NASA/GISS.

First, let’s give the data a little bit of context.  The IPCC reported last year that current climate models are predicting that global temperatures will rise to 3-4°C above preindustrial levels, if the status quo with regards to energy emissions continues unabated, by the end of this century.  But as Hansen and his colleagues pointed out in an excellent (and very readable) 2017 paper called Young people’s burden: requirement of negative CO2 emissions, global temperatures have been increasing at an essentially linear rate of approximately 0.18°C per decade since 1970 (see Figure 2b in that paper).  The linearity is easily observed when you average out the temperatures using a 132-month running mean; this smooths out the natural variabilities due to the El Niño cycle, the solar cycle, and other factors.  With temperatures currently at 1.2°C above the mean temperature between 1880 and 1920 (identified as a reasonable estimate of ‘pre-industrial’ temperature), the current linear trend would place global temperatures at about 2.6°C above preindustrial levels in 2100. What this tells you is that the models expect the rate of temperature increase to accelerate if global CO2 omissions are not sufficiently curtailed.

And unfortunately, this is already starting to be seen in the data (see Figure 4 from Hansen's recent report).  According to the NASA/GISS temperature record, the global mean temperatures of the last five years are all above the linear trendline from Hansen’s 2017 paper. This result was predictable for the strong El Niño years of 2015 and 2016, and even for the moderate El Niño year of 2019. But the large temperature drop in 2018, as the El Niño segued into a strong La Niña event, still produced a mean yearly temperature above what would be expected from the trend. Does this prove that the trend is no longer holding? More data from the next few years will be necessary to confirm that. But the observations are fully in line with what models have predicted, and I would argue that the best guess right now is that the rate of increase in global temperatures is indeed accelerating.

So what does that mean? A continuation of the linear trend would have global temperatures a little higher than 1.3°C above preindustrial levels by 2030. I'm beginning to think that temperatures will be closer to 1.4°C above preindustrial levels by the end of this decade, and that it will be very difficult to avoid crossing the 1.5°C threshold by 2035.  Obviously how much CO2 we collectively emit in the meantime will matter, but it will take a massive effort to change our emissions that dramatically in that small a period of time.

As for 2020, a La Niña event does not appear imminent, so this year should be mostly neutral.  This neutrality should make this year a good barometer for what the current state of the climate really is.  I would expect the mean temperature for the year to hover at or bit below 1.2°C above pre-Industrial levels, making it the third or fourth warmest year in the temperature record.  Anything more than that would be very concerning.

Greetings from the Climate Strike

 This photo is from Greta Thunberg's Facebook page.

This past Friday, September 20, 2019, a youth-led climate strike took place in cities across the world.  Several million people globally, including up to a quarter million people (according to organizer estimates) in New York City, took to the streets to demand that world leaders respond to climate change and global warming with the urgency that science indicates is necessary.  As one of the massive army in New York, I was impressed and inspired by the energy of the protesters and their optimism in their own ability to make change.  But as an atmospheric scientist who has been studying the physics of climate for longer than most of my fellow protesters have been alive, I felt a tinge of sadness that it had come to this.

As sixteen-year-old Swedish activist Greta Thunberg pointed out in her speech to the United Nations on September 23, "For more than 30 years, the science has been crystal clear.”  The climate science community have gathered quite a bit more data since Dr. James Hansen’s 1988 visit to the U. S. Senate made “global warming” a household term, but the data have reinforced the conclusions, not altered them.  The Earth is getting warmer, at a rate which cannot be explained by any known natural cycles.  It can be largely explained, however, by the rapid, steady increase in atmospheric concentrations of carbon dioxide, instigated by the burning of coal and hydrocarbons for energy and aggravated by the loss of forests which serve as the Earth’s primary natural “sink” for the gas.  This is because carbon dioxide absorbs the kind of infrared radiation that the Earth emits, so that the energy contained in that radiation stays in the atmosphere and does not escape into space.  This warming will have consequences, including rising sea levels, stronger storms, and longer and more deadly heat waves.  And the cost of accepting these consequences will be far greater than the cost of preventing them would be.  None of this is new.  None of this is even up for debate on the grounds of reasonable doubt.  And yet, for as long as I've studied atmospheric science, I’ve felt that I was banging my head on a brick wall every time the subject came up in conversation.  I’m not sure that I’ve changed the mind of even one of my skeptical friends and relatives.  And people who have no interest in even acknowledging the existence of global warming, much less doing anything about it, continue to win far too many elections.  But young people have begun to realize that the consequences of global warming will affect them more than they will affect people my age, and they have started listening.  As Thunburg told Congress last week, “I don't want you to listen to me.  I want you to listen to the scientists.”  It would have been self-defeating, on many levels, for me NOT to go and lend my support.

The quote on the poster is from Dr. Seuss's "The Lorax"

I decided to wear a T-shirt I have that says “Ask me a question, I am a scientist.”  I got that shirt volunteering at the Long Island Mini Maker Faire in Port Jefferson, NY this past June.  As I said when I started this blog, one very useful thing we can do as individuals to combat global warming is simply to talk about it, in order to raise awareness and improve understanding.  I figured that shirt would be a good conversation starter.  And indeed, both before and after the march, I had a number of people come up and ask me questions.  Most of the questions were some variation of, “How screwed are we?”  That's not an easy question to answer, because how the Earth looks a century or two from now will depend quite a bit on our actions in the meantime.  The present politically-established benchmarks are not adequate to prevent massive long-term damage.  The Paris Agreement talks about limiting warming to 1.5°C above pre-Industrial levels. But the last time the Earth’s temperatures were as warm as they are now (about 1ºC above pre-Industrial level) for a significant length of time, sea levels were 20 to 30 feet higher.  And the last time levels of carbon dioxide in the atmosphere were as high as they are now, sea levels were 50 to 60 feet higher.  That's the world our descendants would inherit if the current temperature or carbon dioxide level is maintained for a century and beyond — but of course we’re nowhere near leveling off anything.  The good news is that it doesn't have to be that way.  Clean energy is cheaper right now than most people realize, and the technology is improving and will continue to improve if politics allow it.  But, consequences are happening right now.  Sea levels have risen about 8 inches on average globally since the late 1800s, and about a foot in New York City.  That foot mattered when Superstorm Sandy hit my area in 2012.  And those extra inches mattered in the Bahamas this past month when Hurricane Dorian hit.  So did the increasing intensity of hurricanes — the Atlantic used to see a category 5 hurricane once every three years on average, but this is the fourth straight year we have seen one.  As of September 23, 53 people are confirmed dead because of Dorian, but over 1000 are still missing.  And the fierce heatwave in Europe this summer killed 1500 people in France alone.  It didn’t have to be that many, and yet that won’t be the worst heatwave that young or even middle-aged people in France will live through.  The rate of sea level rise has begun to accelerate, and is projected to accelerate further.  It may take a while to level off, even if we do start to act decisively; a just-published paper by a former colleague of mine concluded that it would take twenty years to slow down (much less stop) the rate of temperature increase even with a quick phaseout of fossil fuels.  And temperatures will have to go down before sea level rise can be reversed.  So lives hang in the balance right now, and the number of people killed, or displaced, or pushed from the category of people who “have” to people who “have lost,” will grow and grow.  Unless.

Taken from this article.

From Greta Thunberg's Facebook page.

Here on Long Island where I live, I have friends who write and sing children’s music.  My favorite song of theirs, titled “One Drop in a Bucket,” talks about how one seemingly small action can cascade into something much larger.  I wonder if Greta Thunberg, when she first showed up outside the Swedish Parliament on a Friday last August carrying her now-iconic poster, had any idea that millions of children would eventually join her.  Or that her simple action would be chapter one in a Tolkienesque quest that continues for her thirteen months later.  But that’s what happened.  For the first time in my professional life, I know that there is a large group of people who take the issue of climate change and global warming as seriously as it needs to be taken.  And to those people I can only say, “thank you.”  I’m sorry it has been left to your generation, and I take some responsibility for the fact that we have not done enough, nor have we really tried.  But I’m still here, and I promise to keep banging my head on as many brick walls as I come to, for as long as it takes.

(For updates on new posts, please click the "Follow" button.)

Hope for Renewables

One of the biggest obstacles that renewable energy needs to overcome, in order to provide the lion’s share of the world’s electricity needs, is that it delivers power intermittently.  Solar and wind do not generally provide maximum power at the times that power is needed most, nor can they be counted on to provide power steadily throughout the course of a full day.  And yet the share of renewables in the energy market is growing, largely to this point for economic reasons.  The most recent report on the levelized cost of energy by the investment banking firm Lazard has the price of new utility-scale solar ranging from $36-46 per megawatt-hour (MWh) of energy produced, while onshore wind varies from $26-56 per MWh.  Compare this to the $41-74 per MWh for new natural gas plants, and especially the $60-$143 per MWh for new coal, and the appeal of renewables becomes obvious even before taking environmental factors into account.  According to a recent report published by the Federal Energy Regulatory Commission, the combined capacity of renewables (including hydroelectric, biomass, and geothermal) eclipsed that of coal in this country for the first time in April 2019.  Solar and wind do not yet control a big share of the energy market.  But that will change, perhaps sooner than you think, if the cost of wind and utility-scale solar continue to trend downward.  Plus, transitioning to a clean energy economy will need to be done with more urgency if the United States and the other nations of the world wish to meet their commitments to the Paris Agreement — assuming that those commitments are even sufficient to stop the effects of global warming from causing significant damage and upheaval on a global scale.  

There are basically three ways to address the intermittency issue if the goal is to reduce carbon dioxide emissions in the power generation sector.  One option is to maintain a significant supply of natural gas to run during times that solar and wind are not generating sufficient power.  But even though natural gas is relatively cheap and emits about half as much carbon dioxide per energy released than coal does, it is not likely that the Paris goals can be reached if emissions in the electricity sector remain substantial.  (Keep in mind that clean transportation presently appears to be a more distant objective that clean electrical power is.)  Another possibility is the continued use of nuclear power, including the construction of new nuclear plants.  Nuclear power does not produce greenhouse emissions, but it is beset by economic issues — according to Lazard, the cost of a new nuclear plants would range from $112-189 per MWh.  Ironically, nuclear plants are actually the cheapest type of power plant to operate once the cost of construction has been fully accounted for (only $28 per MWh, compared to $36 per MWh for coal), which means the bulk of their cost comes from their construction.  According to a recent report by the Union of Concerned Scientists, a lot of existing nuclear power plants struggle to achieve cost-competitiveness largely because the companies who operate them are still paying off the construction cost.  The third option is to have renewables produce excess energy at the times of maximum power generation, and then store this excess in batteries.  This option often gets downplayed because it's still fairly expensive; a Bloomberg report from this spring set the levelized cost of battery storage at $189 per MWh in the spring of 2019.  But that price is 35% lower than it was in the spring of 2018, and there are markets right now where renewables with battery storage are cost-competitive, even without subsidies.  Furthermore, a 2018 report from the World Economic Forum suggested that by 2028, the cost of renewables plus battery could be lower than the cost of gas “peakers” in the grid.  And on July 1, Forbes magazine reported on a deal struck by Los Angeles Power and Water to create the largest and cheapest solar-plus-battery project in the world.  The solar power will be delivered at roughly 2 cents per kilowatt-hour ($20 per MWh), with an additional cost of only 1.3 cents per kilowatt-hour ($13 per MWh) for the battery storage.  This combination comes in below typical cost for both natural gas and nuclear power, and it sets the price bar for renewables plus storage that much lower.  It's tough to guess how profitable this plant will be in the long run, but it indicates considerable optimism in the direction the price of renewables with battery storage is going.

In an ideal world, we would collectively aspire to reduce carbon dioxide emissions in electricity generation as quickly as possible.  I’ve discussed a bunch of the reasons for that here on this blog, and continuing to debate what scientists have accepted for over forty years belies the urgency needed to tackle the problem.  People do have a right to be concerned that aggressive reductions will create economic setbacks that will ultimately do more harm than good, but evidence for that being the outcome does not actually exist.  Instead, the Fourth National Climate Assessment of the U. S. Global Change Research Program concluded that limiting global warming to 1.5ºC above pre-Industrial levels as opposed to 2.0ºC would save the American economy half a trillion dollars annually. And the Office of Management and Budget under the Obama administration estimated the economic damage due to global warming, simply in terms of lost federal revenue, could range from $340 billion to $690 billion annually if the status quo in the energy sector continues to hold and global temperatures rise to 4ºC above pre-Industrial levels..  But economics can certainly influence the direction that greening our energy sector takes.  Nuclear power does not leave a carbon footprint, but the cost of constructing new plants remains prohibitive and is not likely to come down.  The cost of battery storage is generally higher for now, but it is trending in the right direction, there is some justification for optimism long-term, and one present development suggests that the cost decline is occurring at a faster rate than projections from last fall and even this spring were suggesting.  And in the meantime, new solar and wind farms are cheaper in most places than new coal and gas plants are; in fact, there are already places in this country where it would save money right now to replace an operational coal plant with renewables.  This doesn’t mean that a major shift to renewables will be easy; in fact, a report out just this week suggests that investment in renewables is not rising but declining.  Present economics does not justify this reticence, though, and environmental concerns clearly demand moving strongly forward.

(For updates on new posts, please click the "Follow" button.)

Remembering Wally Broecker

Dr. Wallace Broecker, a Columbia University geophysicist and climate scientist, passed away this week at the age of 87.  He served as a professor at Columbia for over half a century and published over five-hundred papers in the field.  Broecker is best known, though, for the 1975 paper he published in Science called “Climate Change: Are We on the Brink of a Pronounced Global Warming?”  This paper is significant not simply for the fact that it inserted the term “global warming” into the middle of discussions of the Earth’s climate, first with scientists and then eventually with the public at large, but also because it largely resolved the debate going on in the climate science community about what was happening to the Earth’s climate and what the ultimate cause of that was.

Scientists had been aware, at least in an abstract sense, that carbon dioxide was capable of warming the atmosphere and that levels of it in the atmosphere were increasing  since the early part of the twentieth century.  However, it wasn't until 1958 that amounts of carbon dioxide in the atmosphere were first quantified.  The primary instrument to do this was set up on the summit of Mauna Loa in Hawaii by a scientist named Charles Keeling.  It became clear from Keeling’s data by the mid-1960s that CO2 levels were rising sharply, and that the rate of increase was accelerating.   Global temperature records were sufficiently well-established at that point in time for scientists to know that the planet on a whole had been cooling for the previous three decades, however.  Reconciling the rising CO2 levels and the greenhouse effect with cooling temperatures proved to be a challenge.  In fact, a paper published by two scientists from NASA/GISS in 1971 suggested that the cooling effect of aerosols was overwhelming the warming effect of carbon dioxide, and that humanity was running the risk of plunging the planet into a new Ice Age.  That paper turned out to be flawed, but a new scientific technique revealed that the cooling at the time was primarily the result not of human activity but a natural cycle. Ice and sediment cores were being used to estimate global temperatures in the recent and distant geologic past.  An eighty-year cycle of global warming and cooling found in this data could explain the cooling between the 1930s and the 1960s better than increases in global aerosol amounts could.

This is where Broecker’s famous 1975 paper comes in.  Using a computer model, Broecker was able to show that the natural cycle observed in the sediment cores, superimposed with the warming influence of carbon dioxide, could explain what had been observed in the temperature record up to that point.  But the model did not stop in 1975.  It also predicted that, as the cooling cycle was about to reverse itself and the warming effect of CO2 was strengthening steadily, a sharp and clearly discernible global temperature increase was imminent.  “Once this happens,” Broecker wrote in the abstract, the exponential rise in the atmospheric carbon dioxide content will tend to become a significant factor and by early in the next century will have driven the mean planetary temperature beyond the limits experienced during the last 1000 years.”  That actually wound up happening in the late 1990s, but it had become evident by the early 1980s that Broecker’s predictions were coming true.  As a result, the consequences of human activity on the Earth's climate became very clear to nearly the entire climate science community.

I only met Wally Broecker once myself.  He worked at Columbia, but he was mainly based at the Lamont-Doherty Earth Observatory in Palisades, NY and didn't come down to the Manhattan campus that often.  As my major was applied physics rather than earth science, I didn't go up to Lamont very often, either.  When I did meet him, at a talk he gave at GISS in 1996, I found him to be a very warm, friendly, and outgoing person with an accessible and approachable personality.  I was new to GISS at the time, but it was clear that everybody in the room looked up to him, and that was good enough for me.  He had a remarkable life and career, and while he may no longer be with us, his work remains an essential part of a critically important science.

(For updates on new posts, please click the "Follow" button.)

The IPCC Special Report

The Intergovernmental Panel on Climate Change (IPCC) was created by the United Nations in 1988, as climate scientists worldwide started to lobby for more action on global warming.  Every five or six years, the IPCC releases an Assessment Report consisting of three 1000-page books presenting the present state of the physical science, the impacts and how to adapt to them, and how to mitigate the impacts.  The fifth and most recent Assessment Report was completed in 2014.  The IPCC does not fund climate research, but it does report on papers that have been published and summarizes the results in an organized manner so that the general public can access the information.  The Paris Agreement in 2015 commissioned the IPCC to look at the effects of a world where global warming reached 1.5ºC (2.7ºF) above pre-industrial levels, and how different the world would be if the warming stopped at 2ºC (3.6ºF) instead.  On October 6, the IPCC released a special report called Global Warming of 1.5 °C: an IPCC special report on the impacts of global warming of 1.5 °C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty.  As with all the IPCC reports, it was accompanied by a summary for policymakers, which I am summarizing here.  Policymakers in democracies are ultimately answerable to voters, of course, so it is important that voters understand the general conclusions as well.

The report begins by stating that “Human activities are estimated to have caused approximately 1.0°C of global warming above pre-industrial levels, with a likely range of 0.8°C to 1.2°C. Global warming is likely to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate.”  This estimate is comparable to what has been published in the relatively recent past.  For example, a 2017 paper written by a team led by James Hansen showed that the rate of temperature increase, using an eleven-year running mean to smooth out natural variability, is essentially linear since 1970 at a value of 0.18ºC per decade.  A continuation of this rate would put the world over the 1.5ºC threshold in the 2040s and over 2.5ºC by the end of the century, but as Hansen further explained this past month, there are reasons to think the rate of warming will accelerate in the meantime.  Why do these numbers matter?  The effects of continued warming are potentially numerous.  While some of these remain a subject of ongoing scientific debate, we can talk about rising sea levels with a high amount of confidence because we know where these sea levels were the last time the Earth experienced similar temperatures.  The last time the Earth was as warm as it is now, for example, was during the Eemian interglacial Period from 130,000 to 115,000 years ago.  Sea levels then rose 6 to 9 meters, or about 20 to 30 feet, higher than they are now.  I live on Long Island, to the east of New York City.  There would not be much of Long Island left if the oceans rose that high.  But if the temperature increase does accelerate without being checked, temperatures could reach 3ºC (5.4ºF) above pre-industrial levels by 2100.  The last time temperatures were this high was during the Pliocene Epoch, about three million years ago, and sea levels were about 25 m (80 feet) higher then. Sea levels would take quite a bit of time (centuries at least) to rise by that much, but if warmer temperatures are sustained for a prolonged period of time, the rise will become progressively harder to stop.  And given the number of people on the world who live close to a coast, the degree of upheaval will be massive.

But is there still some hope of keeping temperatures from rising above 1.5ºC, or at least 2ºC?  According to the recent report, “warming from anthropogenic emissions from the pre-industrial period to the present will persist for centuries to millennia and will continue to cause further long-term changes in the climate system, such as sea level rise, with associated impacts (high confidence), but these emissions alone are unlikely to cause global warming of 1.5°C (medium confidence).”  Some degree of continued warming is inevitable regardless of how quickly carbon dioxide emissions are reduced, mainly because carbon dioxide has a half-life in the atmosphere of about fifty years.  In other words, half the carbon dioxide emitted in 1968 is still in the air today.  So it will take time to clean up the atmosphere fully even in the best case scenario.  But the best case scenario can still keep warming under 1.5ºC — provided that the people of the world act with a sense of urgency.

The report discusses a series of emissions reduction scenarios, along with their likelihood of getting global temperatures to stabilize with warming below 1.5ºC by 2100.  The presented scenario that keeps warming under the Paris Agreement’s preferred limit involves reducing global carbon dioxide emissions to a net of zero before 2055, while starting to aggressively reduce the warming from other sources by 2030.  Black carbon (soot) aerosols and gases like methane do not contribute as much as carbon dioxide to global warming, but they still contribute a substantial amount and can not be overlooked.

What kind of sea level rise can we expect to see in the short term?  “By 2100, global mean sea level rise is projected to be around 0.1 meter lower with global warming of 1.5°C compared to 2°C (medium confidence). Sea level will continue to rise well beyond 2100 (high confidence), and the magnitude and rate of this rise depends on future emission pathways. A slower rate of sea level rise enables greater opportunities for adaptation in the human and ecological systems of small islands, low-lying coastal areas and deltas (medium confidence).”  The error bars are substantial with sea level rise, but two points need to be made.  The first is that some degree of rise is inevitable, as the ice sheets are still adjusting to the warming that has already happened.  The second is that the amount of sea level rise will ultimately depend on the temperature at which the Earth stabilizes, and the time it takes to get to stabilization.

What other changes besides sea level can we expect to see in a warming world, and how much difference will limiting the warming to 1.5ºC make compared to a warming of 2ºC?  The report states that “climate models project robust differences in regional climate characteristics between present-day and global warming of 1.5°C, and between 1.5°C and 2°C. These differences include increases in: mean temperature in most land and ocean regions (high confidence), hot extremes in most inhabited regions (high confidence), heavy precipitation in several regions (medium confidence), and the probability of drought and precipitation deficits in some regions (medium confidence).”  This point deals with changes that are already being observed, including in the northeastern United States: the entire northeast has seen an increase in precipitation over the last 30 years relative to the first half of the 1900s, and there are large stretches of New England where the increase exceeds 10%.  The physical explanation for this is that higher temperatures mean more water vapor gets evaporated, and more water vapor going up means more precipitation coming down.  These trends will strengthen as the Earth gets warmer, but can be mitigated if the temperature is stabilized.  Other things that will increase at a temperature increase of 1.5ºC, and even more at 2.0ºC, include impacts on biodiversity and ecosystems, ocean temperature and acidity, and climate-related risks to health, livelihoods, food security, water supply, human security, and economic growth.  And the difficulty of adapting to the changes, naturally, will increase as well.

One graph in the report talks about the risks that are involved to “people, economies and ecosystems across sectors and regions” as the world warms.  Some risks are closer than others; for example, the risk of severe impacts and risks to warm water corals is already high and almost certain to become very high.  The risk of coastal flooding already becomes high with 1.5ºC warming, and the risk of major flooding from rivers becomes high with a warming of 2ºC.  The risk of severe damage to crop yields and heat-related mortality also becomes high at a warming of 2ºC.

While the IPCC acknowledges that any pathway to keeping warming below 1.5ºC will require swift and definitive action, the report proposes several different pathways that are possible.  One involves major innovations in efficiency that enable rapid decarbonization and make it possible to reduce carbon dioxide levels in the atmosphere simply by adding trees.  The second involves a general change in people’s consumption patterns, along with technological innovations in artificial means of removing carbon from the atmosphere.    The third is a combination of the two, and the fourth is a high initial overshoot of 1.5ºC that is overcome by technological innovations in carbon removal.  “Avoiding overshoot and reliance on future large-scale deployment of carbon dioxide removal (CDR) can only be achieved if global CO2 emissions start to decline well before 2030 (high confidence).”  As Benjamin Franklin put it, a stitch in time saves nine.  The sooner we act to control temperatures, the easier it will be.  And it is also much cheaper to plant a tree than to invest heavily in new technology and hope it quickly reaches a point where it can work on a global scale.  

The catch in all this, of course, is that forests require land, cheap solar and wind power require land (at least at present), and food for a growing population rapidly approaching eight billion people requires land.  The world will need to walk a very delicate tightrope.  The task is daunting, and will require creative thinking on the part of many people across the globe.  It is still very possible, though, given sufficient will.

(For updates on new posts, please click the "Follow" button.)

The debate over Antarctic Ice

Scientists know with plenty of confidence what is happening to the average temperature at the Earth’s surface, but the effects of global warming at specific locations are not always understood as clearly.  One location that continues to be a subject of extensive research and considerable debate is the continent of Antarctica.  You might expect that a large landmass covered with ice would be experiencing uniformly large losses of ice in a warming world, but the situation is more complicated and nuanced than that.  The most recent (2013) report of the Intergovernmental Panel on Climate Change (IPCC), the international body that reports on the state of climate science to world governments and to the public, concluded with high confidence that the Antarctic Ice Sheet on the whole has been losing ice.  The ice loss comes primarily from the Northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica (see Figure 1, taken from the IPCC report Climate Change 2013: The Physical Science Basis).  As a consequence, the lost ice is contributing to an acceleration in the rise of seas levels globally.  Note, however, that much of the land mass of Antarctica appears to be gaining some ice, and some sections of the continent are not presently warming.  This conclusion represented the majority of the studies that measured ice changes in Antarctica, but a 2015 study based on satellite data drew a very different conclusion.  The debate over the difference in observed results continues three years later, but a pair of significant papers that came out in June might point toward a resolution.

Figure 1.  West Antarctica is the left side of this illustration, with the Northern Antarctic Peninsula sticking up on the top left.

The major disruption to the consensus opinion of the Antarctic Ice Sheets came from a paper by a research team based at the NASA Goddard Space Flight Center (NASA/GSFC) in Greenbelt, MD.  Led by Jay Zwally, who in 2002 had published a significant paper about the disintegration of the Greenland Ice Sheet, the team looked at data that appeared to show that that Antarctic Ice Sheet was in fact gaining more mass than it was losing.  Much of the data for the study came from the Ice, Cloud, and Land Elevation Satellite (ICESat), a NASA satellite that operated between 2003 and 2009 and used the reflection of laser light to measure changes in height on the Earth’s surface, but other satellite data were used as well.  The data indicated that the height of the ice sheets is rising, not falling, and Zwally and his team concluded from this that the Antarctic Ice Sheets were experiencing a net gain in mass.  This conclusion caused a bit of a stir, to say the least.  Many climate change skeptics posted articles like this one about the result, taking it as proof that global warming was not as bad as “alarmists” in the IPCC were suggesting.  Zwally himself objected to his team’s research being used in such a matter.  At any rate, if the conclusions of the paper held up, it would not change the rate of observed sea level rise; it would, as commentator Jonathan Bamber from the University of Bristol pointed out in a guest post for the RealClimate blog, “make closing the sea level budget a whole lot harder (that is, making the sum of the sinks and sources match the observed rate of sea level rise).”  Ultimately, what that meant was that more research was necessary to understand what caused the discrepancies between what the plurailty of research papers cited in the IPCC had concluded and what Zwally and his team determined from their satellite data.

In their June 14 issue this year, Nature (the UK’s leading weekly magazine on news and major findings within the scientific community) presented a series of papers on Antarctica.  Included in this series was “Mass balance of the Antarctic Ice Sheet from 1992 to 2017,” written collaboratively by a very large number of scientists working under the banner of the Ice sheet Mass Balance Inter-comparison Exercise (IMBIE).  The IMBIE team compiled 24 different estimates of ice sheet balance.  Seven used satellite measurements of surface height, like the Zwally study did, while fifteen used measurements of the strength of the Earth’s gravity at different locations.  The remaining two used the input-output method, where measurements of incoming snowfall are compared against the size of icebergs that have broken off.  The consensus conclusion was that Antarctica has lost ice after all, causing a sea level rise of 7.6 ± 3.9 millimeters globally over the last 25 years.  The findings of the Zwally paper were acknowledged in the appendix, but it was clear that the Zwally paper is an outlier compared to the other studies.

But if the Zwally study’s status as an outlier comes from something that makes its conclusions inaccurate, it still needs to be determined what the source of that inaccuracy may be.  The week after the IMBIE paper was published in Nature, Science (a major weekly publication like Nature, put out by the American Association for the Advancement of Science) published an article from a research team lead by Valentina Barletta of the Technical University of Denmark that looked at how bedrock in West Antartica rises as the weight of the ice above it diminishes due to melting.  (The weight of the ice puts enough pressure on the underlying rock to make it compress, and removing some of the ice causes the rock to decompress.  Both present-day ice melting, and past melting of ice as the Earth emerged from the last Ice Age, can contribute to present-day decompression.)  The team put GPS devices at the top of the bedrock at six different stations in West Antarctica, and found that the bedrock was rising at an average rate of 41 mm per year — much higher than had been anticipated.  This finding suggests that ice mass loss in the West Antarctic Ice Sheet to date has been underestimated.  But on a more optimistic note, it also suggests that the sheet is more stable than previous thought, and less likely to shed a vary large amount of ice into the world’s oceans in a quick but catastrophic collapse.

So how do the two papers tie in to each other?  Zwally’s paper used a series of different values for the rate of bedrock rise, obtained from previous studies at different locations in Antarctica.  For West Antarctica as a whole, the rate of rise cited by Zwally was 26 mm per year.  If the Baretta study’s findings are correct, then the Zwally paper underestimated the bedrock rise by 15 mm/year.  Consequently, in order for the ice height measurements to be correct, an additional 15 mm per year of ice would be melting (or not accumulating in the first place) in West Antarctica.  Now there are some caveats here.  The Baretta study only examined a narrow part of Antarctica, not the continent as a whole.  Further studies on bedrock rise in the rest of the continent are likely forthcoming, however, and if they show that the Zwally study used estimates for the bedrock rise that are too low across the whole of Antarctica, then the conclusions of that paper can be reconciled with other research without needing to explain an error in either the ice height measurements from ICESat or any of the other data that resulted in different conclusions.

The study of the ice sheets in Antarctica is ongoing.  While apparent discrepancies in the findings of different research groups using different methods cannot presently be definitively explained, there is reason to hope that this puzzle will be solved in the relatively near future.

(For updates on new posts, please click the "Follow" button.)