This chart perfectly explains what’s at stake in the quest to stop climate change

פורסם: 16 ביולי 2016, 2:23 על ידי: Sustainability Org   [ עודכן 16 ביולי 2016, 2:24 ]
By Chris Mooney June 23 2016

This image provided by NASA shows Arctic sea ice at it maximum, the lowest on record. The winter maximum level of Arctic sea ice shrank to the smallest on record, thanks to extraordinarily warm temperatures, federal scientists said. The National Snow and Ice Data Center says sea ice spread to a maximum of 5.607 million square miles in 2016. That’s 5,000 square miles less than the old record set in 2015, a difference slightly smaller than the state of Connecticut. (NASA via AP)

Here at the Energy and Environment blog, we cover, regularly, the tipping points of climate change — how, for instance, the glaciers of West Antarctica may already have passed a key threshold that leads to unstoppable melt.

We cover the history of the Earth’s climate — including why the Holocene era, which began some 11,700 years ago and we lived in up until fairly recently (when many researchers believe an “Anthropocene” began), was so stable and conducive to human civilization.

And of course we cover the quest to keep warming below the Paris climate targets, 1.5 degrees and 2 degrees Celsius, and the scenarios for greenhouse gas emissions that might be capable of doing that — and also those that can’t.

[The math the planet relies on isn’t adding up right now]

But these are all complicated, nuanced stories, and the idea that they can all be pulled together into one analysis — much less one figure — is hard to believe.

Nonetheless, I think three researchers from the Potsdam Institute for Climate Impact Research have done precisely that with the image below, which is part of an optimistic essay they just published in Nature Climate Change, suggesting that the recently negotiated Paris climate agreement has what it takes to stabilize climate change. In the process, the researchers deliver the sort of rare big picture analysis that we mortals need now and again to understand why scientists are generally so freaked about a warming climate, and also why, in this case, they’re feeling a shard of hope.

Here’s the image, with unpacking to follow:

Schellnhuber et al., Nature Climate Change.
Schellnhuber et al., Nature Climate Change. The figure shows, according to the authors, the “global mean surface temperature evolution from the Last Glacial maximum through the Holocene,” combined with the temperature range aspired to in the Paris climate agreement, possible temperature rises for different greenhouse gas emissions scenarios (RCP2.6, RCP8.5, and so on), and the tipping point thresholds for various major planetary changes.

“Earth system science has come of age and can provide robust evidence for the intuitive assumption that it is not a good idea to leave the ‘safe operating space’ of humanity, and that this space is well within the Paris confines,” Hans Joachim Schellnhuber, Stefan Rahmstorf and Ricarda Winkelmann of the Potsdam Institute write in the study that contains the image.

“The sustenance of Holocene climate conditions under which Homo sapiens thrived while the Neolithic Revolution established human civilization was an early key argument in favour of the 2 °C guardrail,” they further note. Both quotations summarize the key point of the image: The Paris agreement, if implemented successfully with its targets respected — and especially the lower boundary, 1.5 degrees C —  gives hope of keeping us from exceeding tipping points that would erase the key climatic parameters that sustained our very development.

That’s a pretty big deal.

To show this, the graphic blends together four key elements — where current temperatures are in the context of the last 22,000 years, including the stable period that fostered modern humanity; where the Paris climate agreement aspires to hold temperatures and how that fits into this context; how high temperatures could rise under various greenhouse gas emissions scenarios; and which major parts of the climate system could be “tipped” at different temperature ranges.

“This graphic is really based basically on the advances made by the entire climate research community over I would say the last two decades,” Winkelmann said in an interview. It “sort of summarizes what we know right now about tipping elements, and how the risk increases with increasing global warming to tip one of these major, major parts of the climate system. And you can really see the tipping elements as the major Achilles heel of the climate system.”

The authors’ argument is not, of course, that we’ll be perfectly safe if the Paris agreement is successfully implemented. We’re already seeing devastating losses to coral reefs — which, the image suggests, are the single most vulnerable element in the entire system — a plunge in Arctic sea ice, and, many scientists think, a tipping point for West Antarctica.

These are among the most sensitive elements of the Earth’s climate system, and as the figure shows, we start fiddling with them even before we enter the Paris temperature range between 1.5 and 2 degrees C.

Nonetheless, if we exceed the Paris boundaries, we appear increasingly likely to commit to irreversible changes in these areas. “Beyond 2 °C the course would be set for a complete deglaciation of the Northern Hemisphere, threatening the survival of many coastal cities and island nations,” they write.

And blundering further, we could subject ourselves to many other risks. Those include, in the figure above, shutting down the ocean’s “thermohaline” or overturning circulation (THC), changing the El Nino-Southern Oscillation (ENSO) in favor of more dominant El Nino conditions, and also tipping the gigantic ice mass of East Antarctica (EAIS). (For a full description of these tipping points or “elements,” see here.)

“There are some that could possibly already be switched within the Paris range, and then there’s another cluster of tipping elements, where we have the chance to avoid tipping those for even higher temperatures,” Winkelmann said. She notes that many of the tipping elements involve a feedback process: Greenland’s ice sheet, for instance, will lose elevation as it melts, subjecting it to warmer temperatures in the lower atmosphere, which will cause more melt. And so on.

Granted, in any given case, some scientists might fear or suspect that some of these tipping points are closer to us than the diagram above suggests. The Amazon, the boreal or northern forest, the Atlantic ocean’s circulation, northern permafrost and East Antarctica are already showing signs of changes. But the researchers think these systems might nonetheless be more resilient.

They go on to argue a more difficult point — that we just might see an “exponential” growth of wind and solar that is sufficient to, in turn, trigger an “implosion” of the fossil fuel industry and thus help keep within the Paris targets. Something at least this dynamic would be needed to get the world to stop emitting fast enough to stay within the temperature “guardrail” above, without relying on pie-in-the-sky technological fixes like so-called negative carbon emissions.

The truth is that renewables are growing very rapidly right now, but from a low level of penetration, and the precise technological evolution of our energy system remains unclear.

Still, nobody can claim that scientists haven’t made clear what’s at stake.

“One could basically say that what we’re doing today with greenhouse gas emissions — which is just basically in a moment when you look at the geophysical timescales — that has consequences for decades, centuries, millennia,” Winkelmann said.