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.

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