There’s been a lot of talk recently about limiting global warming to 1.5°C, mainly focused on two things: 1) how important it is, and 2) how difficult it will be. This raises an important question: how far have we come already, and how much farther until we reach the 1.5°C limit?
The threshhold 1.5°C refers to how much we have warmed the planet above its pre-industrial temperature, but that of course begs the question, what was pre-industrial temperature? It’s easy to say it’s the long-term average around the year 1750 (about the start of the industrial revolution), but we don’t have enough historical thermometer data to know precisely what that was. Various analyses have substituted a different reference level based on a different reference time. For example, some analysis uses the average temperature over the period 1850-1899, some use the average over the period 1861-1880. These aren’t pre-industrial times, but at least they give us a place to start, i.e. somewhere around the late 19th century.
A difficulty using reference periods 1850-1899 or 1861-1880 is that not all global temperature data sets cover that period; in particular, data from NASA and from NOAA don’t start until 1880. There’s also the fact that true pre-industrial was probably cooler than the late 19th century, an issue studied by Schurer et al. specifically in the context of limiting future warming to prominent threshholds (1.5°C or 2°C). They conclude that “pre-industrial” was probably cooler than the late 19th century by as much as 0.2 °C. They also show that even 0.2°C difference can have an important impact on the likelihood of staying below threshholds, and the timing of exceeding them if we do cross those lines.
Another complication is that most estimates of how far we’ve come are based on the global temperature from the Hadley Centre/Climate Research Unit in the U.K. It’s a fine choice but possibly not the best, because by omitting the Arctic (the fastest-warming region on earth) it may underestimate the total temperature increase.
For this blog post, I’m going to set my reference level to the smoothed mean temperature in the year 1900. This is rather like the 1890-1909 average. There are some advantages to this, including the fact that it’s a wee bit cooler than most “late 19th century” averages (but by less than 0.1°C) so it won’t be as much warmer than true “pre-industrial”, and that data coverage is a bit better around 1900 than before 1900 (especially 1850-1899, when data coverage during 1850 is really very poor).
I’ve looked at the five most prominent global temperature data sets, HadCRUT4 (from the Hadley Centre/Climate Research Unit in the U.K.), NASA (GISS, the Goddard Institute for Space Studies), NOAA (National Oceanic and Atmospheric Administration), Cowtan & Way (a modified, and in my opinion improved, version of the HadCRUT4 data), and Berkeley (an independent effort, the Berkeley Earth Surface Temperature project). I’ll smooth each time series (with a modified lowess smooth), then offset them so that the value during the year 1900 is zero (making the 1900 smoothed mean the reference level). Then we’ll look at where we stand, in particular at the smoothed value now so we know how far the trend has gone apart from the never-ending fluctuations.
Here are the smoothed temperature series from each data set:
All are set to a level of zero during the year 1900. That make the value now an estimate of how much we’ve warmed the planet, and here are the numbers I get:
The first thing to note is that for all five data sets, we’ve already heated up the earth by more than 1°C. The least is according to HadCRUT4, only 1.05°C, while the most is from the Berkeley data, 1.19°C. If our goal is to avoid going above 1.5°C, then the Berkeley estimate gives us a third less room to work with than the HadCRUT4 data.
That’s not an insignificant difference. It means that the available carbon budget (how much we can still emit and stay within threshhold) is a third less; a 300 GtCO2 emission limit using HadCRUT4 data is only a 200 GtCO2 emission limit using Berkeley data.
All of which emphasizes just how important it is to begin drastic emissions reduction now.
Even if we do decide on an aggressive emissions reduction plan and actually stick to it, there are other things to worry about. There really are feedbacks in the climate system and some of them might get ugly. Best-known (and probably most-worried-about) is permafrost melt, which threatens to add even more CO2 to the atmosphere than we already have. Perhaps the best approach to these is: don’t poke the bear.
There there are man-made aerosols, which cool the planet, but if we stop emissions we’ll stop most aerosols too and end their cooling influence. Estimates of how much man-made aerosols are cooling the world vary widely, but 0.5°C is a common value. If we eliminate 0.5°C aerosol cooling, we’re over 1.5°C already, but again there are complications and the rapid decay of short-lived greenhouse gases may offset most of that, making the net long-term effect nearly zero. It’s … complicated.
There’s also the fact that most near-term forecasts are based on a linear approximation of the climate system’s response to perturbations. We all know that linear response is usually close and often useful, but we also know that the system isn’t linear, and when the nonlinearity kick in things can get very ugly.
All of which emphasizes just how important it is to begin drastic emissions reduction now. Most of the world seems to be waking up to the fact of just how important and how urgent this is. Unfortunately, the United States is going the wrong way at just the wrong time.
There’s sure to be disagreement about many of my choices and results. But only those in serious denial disagree with how important it is to begin drastic emissions reduction now.
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