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.
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