A major new study has been published – one that provides much more certainty about the extent of future warming we might expect.
Along with many other international climate scientists, it was led by my colleague, climate scientist Steven Sherwood from the University of New South Wales in Australia. So, I asked him a few questions about it, to drill down on what this means for us and the future.
We know that the Earth’s climate is warming as the concentration of greenhouse gases such as carbon dioxide in the atmosphere increases. Since the 1950s, NASA temperature data have shown that Earth has warmed ~ 0.8 ° C until the last decade.
It is also almost certain that mankind is causing this recent warming (as I write about in detail here). But what about future warming? How do climate scientists predict the future?
The great unknowns: energy, economy and politics
The scale of future warming remains uncertain for various reasons, the biggest unknown being how much carbon pollution humans will emit in the coming decades. That is based on political and economic systems – hardly anything we can predict in the coming months – let alone the coming decades!
That said, scientists have developed complex terrestrial systems to predict the future using a variety of future carbon pollution scenarios – ranging from the ‘burn all coal reserves’ option to the ‘shut down all-morning-fired power plant’ option.
But another important element of uncertainty is how sensitive the Earth’s climate is to carbon dioxide.
Scientists call that “equilibrium of climate sensitivity.” It represents the temperature rise for a sustained doubling of carbon dioxide concentrations.
Equivalent climate sensitivity has long been estimated at a probable range of 1.5-4.5 ° C. This means that if / when carbon dioxide in our atmosphere reaches 560 parts per million (ppm), Earth will heat somewhere between 1.5-4, 5 ° C, which has long been frustratingly uncertain.
The new study is the most complete study to date of all available evidence, and finds the most likely range 2.6–3.9 ° C. But this ‘equilibrium temperature’ would last for hundreds of years, says Sherwood:
“It takes a long time to fully adapt to a change in the rate of energy that comes in, hundreds of years. However, most of the warming happens within a decade of the change. We think the actual warming over the next century (given an emissions scenario) is closely related to the amount of equilibrium arousal, so if you know one, you roughly know the other. “
How far along are we today after doubling atmospheric carbon dioxide concentrations? About halfway through. It was ~ 280 ppm before industrialization (AD 1880) and is ~ 413 ppm today ~ 413 ppm today (see where the solid line ends below).
Given a steadily increasing concentration without much political action by the world, this would mean that the concentration would double to 560 ppm by ~ 2070.
That the new study implies that we are likely to be trapped today somewhere between 1.3-2.0 ° C warming in the long run. But there is something else that is changing.
The good news: the extreme scenarios are probably not
Something for a warming of 5-6 ° C by 2100 was not out of the question, even recently – what would cause an enormous catastrophic impact over a large part of the world. The good news from this new work is that the warm-up scenarios with worst cases are off the table, says Sherwood:
By 2100, I think we could exclude about 5 ° C by 2100, assuming the world does not go bonkers, but not 2200 if we continue to burn fossil fuels after the end of the century and beyond.
But 1.5 ° C is gone and probably 2 ° C …
The most optimistic future forecast for the future includes the world by 2050 drastically cutting coal, oil and gas.
But even doing so means that it is almost impossible to stop global warming below 1.5 ° C, says Sherwood:
The most optimistic future scenario would give us a 83 percent chance of staying below 2 ° C, but a 33 percent chance of staying below 1.5 ° C, so staying below 1.5 ° C would be extremely difficult because this scenario fairly extreme measures would be necessary.
The most optimistic scenario does not play out in reality, so the window also quickly closes on limiting heating to 2 ° C given the existing emission trends, says Sherwood:
A scenario close to what we expect under current global policies gives us less than a 10 percent chance of staying below 2 ° C. So, in fact, we need to step up our efforts and commitments significantly to have a decent chance of reaching the 2 ° C target.
The most likely scenario based on the new study and the most likely future pollution scenario is 2-3 ° C by 2100.
Who is talking about 2-3 ° C? It does not seem much …
Imagine yourself in the middle of lush green Central Park in the middle of New York City. If you could somehow travel 20,000 years back, what would you see if you looked across Manhattan? Thick forests and lakes?
In fact, it would look more like Antarctica – forests replaced by a thick wall of icy ice that encompasses all of New York City, and extends to Canada.
Why is the visualization of the New York City ice sheet important? Because sometimes when climate scientists talk about 2-4 ° C average global warming – it sounds like a summer vacation. But just a drop of ~ 4 ° C ~ 4 ° C in average global temperature was enough to cause that enormous 1 mile thick ice sheet to cover New York.
As a climate scientist, it is difficult to convey this, but small changes in the earth’s average temperature create great influence over long periods of time.
This expert response was published in collaboration with independent fact-checking platform Metafact.io. Subscribe here to their weekly newsletter.
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