In looking at the problem of human-caused climate change, we often miss a critical point. Human greenhouse gas emissions warm the atmosphere, melt ice, and heat the ocean, yes. But a warming world also releases its own additional volumes of carbon, often in the form of methane and CO2. In the Arctic, we have seen massive releases from natural stores of CO2 and methane. Releases that, on their own, are enough to amplify the already significant warming caused by human fossil fuel emissions. Now, according to a new study, the tropics are also contributing an amazing volume of carbon as they continue to endure the insults of warming.
In essence, when humans warm the globe, they are kicking nature. And an insulted nature then releases increasing volumes of her own carbon stores in an ever-worsening cycle until humans stop and the severely riled natural cycle runs its course. Such an agitated carbon cycle can swiftly trip into a phase that accelerates atmospheric heating to rates far more rapid than initially feared. And, as research progresses, it appears the worst fears about nature’s sensitivity to our insults are steadily being realized.
Chasing down the feedbacks
Climate researchers must have felt an ominous sinking feeling when they discovered this year that, as the globe heated up, changes to the world’s clouds resulted in an amplifying feedback to human-caused warming. Many researchers had left previously clouds out of climate models and, as such, the models showed lower rates of warming than what we would find from comparable measures in paleoclimate. For example, many climate models only showed about 1 to 1.5 C warming as a result of atmospheric CO2 levels at 400 parts per million (the level seen today), while paleoclimate showed long term warming twice that, in the range of 2-3 C. This disparity created a bit of cognitive dissonance in the science between those who modeled climate and those who studied past climates. But, slowly, the gap is being bridged and the new research in clouds makes up a portion of this difference. But it was not the only missing feedback.
Another measure some climate models leave out is the complex carbon cycle response from the Earth System as it warms. These include the carbon feedbacks in the Arctic we’ve explored so much on this blog but they also include large systems around the globe that are likely to experience profound change as they warm. (It’s worth noting that some models do include growing portions of these measures, such as the NCAR global climate models which show up to 7 C warming with each doubling of CO2).
Tropics carbon out-gassing found to equal almost 20% of human emissions
(Forest Fires in the Tropics, like these Amazon fires, are one of the many human-driven changes to the region resulting in a 2 gigaton per year annual carbon emission increase over the past 50 years. Image source: Earth Observatory.)
Now, new research from the University of Exeter has discovered that the tropics are also beginning to contribute massive volumes of carbon to the global climate system. In total, the study found that carbon emissions from the tropics had increased by 2 gigatons over the past 50 years in response to human-caused warming. This 2 gigaton tropical emission is just a little less than 20% of the carbon emission coming from the human use of fossil fuels. This amount is a staggering total, roughly equivalent to total US carbon emissions from all CO2 related sources. When combined with the emission already ramping up in the Arctic, this new source is now a very powerful contributor to warming the planet.
The study showed that just one degree of global temperature increase was enough to result in the added flood of this extraordinary volume of carbon. The extra heat particularly impacted rain forest systems which saw the increased insults of added heat, drought and fire.
Paper authors, including Professor Friedlingstein, who is an expert in global carbon cycle studies, attributed a good share of this change to the increasing prevalence of drought which results in both more rapid decomposition and more widespread forest fires.
It is worth noting that tropical systems have been plagued with increasing instances of fire and drought. As an example, the massive forests of the Amazon are under the combined assault of drier conditions, large forest fires, as well as smaller understory fires that eat away at large sections of the Amazon. In a typical year, an area of the Amazon the size of South Carolina is destroyed by small fires alone. And it is mechanisms such as these that were found to increase the rate at which tropical carbon is returned to the atmosphere in a warming climate. Dr. Friedlingstein noted: “Current land carbon cycle models do not show this increase over the last 50 years, perhaps because these models underestimate emerging drought effects on tropical ecosystems.”
(Map of incidents of small fires in the Amazon from 1999-2010. Image source: Earth Observatory.)
The tropical forests are one of the largest carbon sinks on the planet. And climate scientists have long warned that this carbon could be returned to the atmosphere as the Earth was forced to rapidly warm by human emissions during the 21rst Century. Overall, what we are now witnessing is just such a rapid intensification of that transfer. It is one that probably approaches the significance of another major carbon stores transfer emerging in the Arctic. When combined with the massive and still growing human fossil fuel emission, these carbon flows are in the process of tipping the planet into a very hostile and unstable new climate state.