The No. 1 greenhouse gas produced by human activity is carbon dioxide. It had been rising since the industrial revolution, and accelerating in both volume and impacts on a mean warming climate since the 1980s. Now, at terrible cost, these emissions are expected to temporarily drop by about 8% globally this year due to the economic disaster. Early data analysis points to only an extremely small, temporary effect on the rate of warming from this reduction, if any.
At the same time, other changes in nature will begin to lessen the amount of carbon dioxide which can be absorbed by vegetation and the oceans. These changes will be occurring in a longer time frame, so they won’t counter the temporary small reduction in human carbon emissions.
There is growing evidence tropical rainforests are beginning to show signs of becoming carbon sources (sources for additional carbon emissions) instead of performing as carbon sinks (sinks absorb carbon dioxide). If this process continues, it would be the first time in thousands of years the tropical forests switch from carbon sink to source. In an article published by the Yale School of Forestry and Environmental Studies, the changes are being documented.
Atmospheric chemist Luciana Gatti reported to Yale aircraft measurements over the Amazon actually detect more carbon dioxide emission than absorption. Some of this is due to deforestation, mainly in Brazil, but some of it is due to warming feedback already present in the air. In the past, additional carbon dioxide in the air fed more growth in the rainforests. Now, excessive warming from the greenhouse effect has slowed the growth rate of foliage, outstripping its capacity to absorb as a carbon sink.
“We have hit a tipping point,” Gatti said.
Her earlier work had noted these impacts mainly in drought years, when forest fires multiplied. Now, her team has found the same shift from sink to source is being observed in wet years as well. This may be critical in the rate of a mean warming climate going from bad, but more manageable, to worse and much less manageable.
Current climate models work on a premise of stability in the role of tropical rainforests performing as carbon sinks, which is part of the foundation necessary to keep global warming below 2 degrees Celsius. Beyond 2 degrees warming, impacts later in the century (sea level rise, wildfires, expanding drought in some regions, floods in moist regions, etc.) become much more of a threat to the globe’s inhabitants. If this trend continues, current climate forecasts will be underestimating the rate and extent of warming due to the inadequate volume of atmospheric carbon dioxide present in the models. Gaitti said the most immediate step to slow this alarming trend would be for Brazil and other nations to act more aggressively in countering deforestation.
If these measured shifts are occurring in the Amazon, they are happening in other tropical rainforests. As outlined in the Yale publication: “The ability of intact areas of the rainforest to absorb CO2 have already halved since the 1990s, said Carlos Nobre of the University of Sao Paulo, Brazil’s most noted climate scientist. Passing the tipping point for the whole forest would release more than 50 billion tons of carbon, he said recently, which is the equivalent of five years of global fossil fuel and industrial emissions.” Nontropical forests in temperate zones continue to serve as carbon sinks.
Another ongoing (and previously predicted) shift is the warming ocean’s ability to act as a massive carbon sink. The warmer the oceans get, the less carbon and heat will be absorbed by these vast sinks. In this National Oceanic and Atmospheric Administration graph, the rate of global warming sea surface temperatures is apparent.
Interestingly, oceanic warming is faster in the Southern Hemisphere and is responsible for more rapid melting of the Antarctic ice shelves from underneath around the periphery of the continent.
Warming is also accelerating permafrost thaw in the Arctic, releasing huge stores of previously trapped carbon and methane into the atmosphere.
These additional sources for more carbon dioxide in the atmosphere are outstripping earlier climate models’ predictions of carbon volume. It is quite possible the additional carbon will mean previous climate models have been underestimating the scale of future warming this century. If that were the case, it would mean humankind’s efforts to reduce fossil fuel usage would take on more urgency well into the future, long after our current public health and economic disasters wane.
The next generation of climate models, already in the works, will be taking into account the additional carbon dioxide not included in earlier models. From the Yale article: “For the first time, they capture the full range of possibilities for how nature’s ability to soak up CO2 may change as the climate changes, said Richard Betts of Britain’s Met Office Hadley Centre, one of the world’s top climate modeling groups. His initial assessment of the early outcomes of these new models is sounding alarm bells.”
The next problem to recalculate is the volume of powerful greenhouse gas methane in the atmosphere. Originally, the total amounts were expected to remain stable. Now, it is known that more trapped methane from ocean beds, agriculture, fracking and the melting permafrost is being released, but more reliable estimates on future volume of methane releases are not yet available for the new models.