Understanding Our Carbon Budget for 1.5 Degrees C of Warming

By: Chandu Visweswariah

Graphic by Prof. Rob Jackson of Stanford

A very interesting paper entitled “An assessment of Earth’s climate sensitivity using multiple lines of evidence” was published in Reviews of Geophysics on July 22, 2020 and discussed in the New York Times.  The paper’s lead author is Prof. S. Sherwood of the University of New South Wales in Australia, with 24 other scientists as co-authors.  The paper studies climate sensitivity which is defined as the amount of global warming due to doubling of carbon dioxide levels from the pre-industrial era.

The critical result is captured in the graphic on the right.  Since 1979, it has been believed that the range of global warming due to a doubling of CO2 is 1.5 to 4.5 degrees Centigrade, with fairly long tails of low probability regions.  This paper has narrowed the likely distribution to 2.6 to 4.1C.

Source: Sherwood et al in Reviews of Geophysics, IPCC. By: The New York Times

So, what does this mean?  For one, it means that scientists are now able to be more precise in projecting the amount of global warming for a given level of CO2 increase in the atmosphere.  Second, the newer study is in the “middle” of the previously assumed range, thus both validating and tightening that range, while discounting extreme values as highly improbable.

However, does it mean that we are doomed to a minimum warming of 2.2C?  The simple answer is no, not unless we allow CO2 levels to double since pre-industrial levels.  Right now, we are at about 49% of increase and on a status quo path to double (i.e., 100% increase) by the end of the century.  There is also some good new news in this report on the high end of climate sensitivity: for a doubling of CO2 levels, warming above 4.1C is highly unlikely.

Courtesy Prof. Rob Jackson of Stanford

We know from the 2018 IPCC report that warming above 1.5C exposes us to severe and irreversible climate harm like rising seas, extreme weather, melting polar ice caps, ice-free Arctic summers, bleached coral reefs, etc.  So, what must we do to prevent this harm?  The high-level answer remains the same in light of the new study: we must decarbonize quickly and purposefully! By all measures, our carbon budget to keep warming below 1.5C is running out.  This is best illustrated by means of a carbon budget in the figure on the left, in an animation created by Prof. Rob Jackson of Stanford.

Compared to the pre-industrial era, we have a budget of carbon that will limit warming to 1.5C represented by the bucket in the figure.  We have already filled 91% of the bucket and we continue to emit over 50 billion tons of CO2 annually, with the United States and China playing leading roles today. Without rapid decarbonization (or a big hole in the bucket that can suck CO2 out of the atmosphere), the status quo will make us run out of any remaining carbon budget in 8 to 10 years, meaning that warming will sharply exceed 1.5C.

Can we save ourselves?  Yes, we can, but the rate of necessary decarbonization is high due to years of neglect.  With every passing year, the task only becomes more uphill as shown in the graphic on the right, and wonderfully illustrated in this 12 second video.  The colored lines show that a gentler pace of decarbonization would have been sufficient if we had started our efforts at the turn of the century. Instead, carbon has gone in the wrongdirection and as a result, we need to decarbonize rather abruptly now to stave off the worst effects.

Courtesy UN Environment Programme emissions gap report

How abruptly do we have to decarbonize?  Scientists can now work backwards from 1.5C of warming and translate that into a carbon budget.  That carbon budget is a range, and we are playing a game of roulette in which our odds get better if we can decarbonize more rapidly.  To be extremely likely to limit warming to 1.5C, we need to decarbonize at an astonishing 15% per year (black line in the figure).  To be likely to limit warming to 1.5C, we need to decarbonize 5% per year.  To have a 50-50 chance of limiting warming, we need to decarbonize at about 3.5% a year.

Let’s not forget that all of the above pathways require us as a first step to “bend the curve” and start emitting less than the previous year, not more.  The Croton100 goal of reducing 5% a year, 50% by 2030 and net neutral by 2040 is the most sensible middle-of-the-road goal for preventing environmental catastrophe.  As the figure on carbon pathways illustrates, there is no leeway, there is no time to dither, the time to decarbonize is NOW!

Here are the take-aways from this blog:

1.  The amount by which the earth warms due to greenhouse gases is now better understood.

2.  Our carbon budget to stave off the worst impacts of climate change and limit warming to 1.5C is 91% exhausted.

3.  Status quo means that we will run out of carbon budget in 8 to 10 years (catastrophic).

4.  Status quo also means that carbon in the atmosphere will double by the end of the century and we will see warming in the 2.6 to 4.1C range (also catastrophic).

5.  We need to decarbonize rapidly to be likely to stave off the worst effects: 50% emissions reduction by 2030 and net zero carbon by 2040.

6.  The time to act is NOW!  Today’s emissions damage the earth for a long time!

Even as we bend the curve on COVID, we must simultaneously pay attention to the environmental crisis by bending the carbon curve.

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