Why are positive climate feedbacks so negative?
The "Hothouse Earth" report caught the public's attention. But the coverage doesn't tell the full story.
This story originally published on World Resources Institute.
If your colleague or child does well and you give her or him positive feedback, that’s good.
If climate change causes a cascade of impacts that result in additional climate change — which scientists call "positive feedback" — that’s bad, and maybe catastrophic.
I started researching the bad kind of positive feedback in the 1980s and eventually learned to refer to "amplifying feedbacks" to avoid confusion. A few other scientists also have written about the risk that amplifying feedbacks could make climate change much worse than most climate models predict, but these worries had not received much attention.
That changed recently when a group of 16 prominent climate scientists published a perspective in the Proceedings of the National Academy of Sciences warning:
(S)elf-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a "Hothouse Earth" pathway even as human emissions are reduced.
Why did this paper garner headlines around the world when previous articles on the topic were barely noticed outside a small circle of scientists? Perhaps due to the use of the evocative term Hothouse Earth. Perhaps because climate change is not just a forecast anymore, but something we experience day-to-day. Perhaps because the paper was published while the earth is ablaze. Perhaps because the current political climate puts many people in an apocalyptic mood. Or perhaps all of the above.
In any case, now that people are paying attention, let’s take a closer look at what climate feedbacks are, how they work and what we can do about them.
Amplifying global warming
The most basic and important amplifying climate feedback is the water vapor feedback. As heat-trapping gases such as carbon dioxide are added to the atmosphere, earth’s surface and atmosphere warm up. Warmer air holds more water vapor. But water vapor also traps heat, so the extra water vapor in the air amplifies the initial warming.
Similarly, the ice-albedo feedback amplifies global warming because as the earth warms, sea ice and mountain glaciers melt, exposing darker surfaces underneath. These darker surfaces reduce the amount of sunlight reflected out to space (the earth’s albedo), which means more energy is available to warm the planet, amplifying the initial warming.
This water vapor feedback and the ice-albedo feedback are built into all climate models and are the key to why small additions of carbon dioxide can have such a large impact on our climate. Because these feedbacks are already accounted for, the authors of the Hothouse Earth paper take them for granted and focus on more complicated biogeochemical feedbacks, such as permafrost thawing and reduced uptake of carbon dioxide by land plants and ocean phytoplankton, which are not included in most climate models (see here for a more complete list).
The size of this effect depends on the total amount of organic matter that decomposes and the ratio of methane to carbon dioxide released, as kilogram-for-kilogram methane traps 36 times as much heat as carbon dioxide over a 100-year period.
A relative weakening of carbon uptake by plants and phytoplankton is expected as global warming continues, due to a number of processes, some of which depend on the rate of warming as well as the magnitude. An increase in concentrations of carbon dioxide in the atmosphere by itself can enhance photosynthesis (referred to as carbon dioxide fertilization) and this appears to be absorbing a portion of the carbon dioxide emitted by burning fossil fuels.
As global warming accelerates, however, this effect will weaken and be increasingly counteracted by ecosystem disruptions, such as pine beetle infestations that can decimate forests and the increase in wildfires we are seeing. At the same time, microbial decomposition of soil organic matter accelerates as temperatures rise, returning more carbon dioxide to the atmosphere. In the oceans, global warming reduces upwelling of nutrients and diminishes the strength of the biologic carbon pump, which transfers carbon dioxide from the upper ocean to deeper layers, as phytoplankton take up carbon dioxide, then naturally die and sink.
Hothouse earth vs. stabilized earth
The Hothouse Earth paper estimates the enhanced warming expected from these and three other processes expected by 2100, assuming that global warming otherwise would be 2 degrees C by then. They find that the sum of these effects by 2100 would add about 0.5 C to global warming, with a range of 0.24 to 0.66 C. That sounds bad, but hardly catastrophic. Unfortunately, this isn’t the end of the story.
First of all, it is a mistake to simply add up the effects of the individual feedbacks to estimate their overall impact, because they amplify each other and themselves as well as the physical feedback processes already built in to climate models. This means that the combined effect could be much, much larger than a simple sum, particularly if other feedback effects are near the upper bounds of their uncertainty ranges. These processes take time and much of this further amplification likely would play out after 2100. At that point, other feedback mechanisms which are not expected to be significant during this century could kick in, such as the disintegration of methane hydrates (a crystal structure that traps large amounts of methane under high pressure-low temperature conditions).
The Hothouse Earth scenario reflects the risk that fossil fuel combustion, deforestation and other human activities in the near term could set in motion a cascade of feedback processes that would continue to cause global warming even after these emissions were curtailed, as shown in this graphic from the paper:
Unfortunately, we don’t know exactly where the planetary thresholds lie. Scientists (including the Hothouse Earth authors) believe they are remove carbon dioxide from the atmosphere — which we will need to reach zero net emissions and may need to ramp up quickly if we find ourselves sliding toward Hothouse Earth.
WRI will have more to say about carbon dioxide removal in the coming weeks. Stay tuned.
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