A new research led by scientists at UC Irvine and Boston University, published in Nature Climate Change (Jonathan A. Wang, Alessandro Baccini, Mary Farina, James T. Randerson, Mark A. Friedl, Disturbance suppresses the aboveground carbon sink in North American boreal forests, Nature Climate Change, 2021; DOI: 10.1038/s41558-021-01027-4 ) suggests that all the new green biomass is not as large a carbon sink as scientists had hoped.

There was a hope that as more plants start to grow in the Arctic and boreal latitudes as our warming climate makes those regions more hospitable for plants, those photosynthesizing plants would work to help sequester the atmospheric carbon dioxide that helped them flourish in the first place.

“What does greening really mean? Can we really trust it to save us from climate change?” said Jon Wang, an Earth system scientist at UCI who the led the work alongside BU Earth & Environment professor Mark Friedl. “A big question is: What will happen to the carbon that is currently stored in these forests as above-ground biomass in the face of a changing climate?”

The answer is that a lot of the carbon is not staying stored in the plants. Because as fires and timber harvests at those latitudes become more and more common as climate change makes those parts of the world hotter and drier and more arable at rates sometimes twice that seen at lower latitudes, much of the new green biomass is not storing carbon – it is combusting during wildfires.

“What we found overall is across this whole domain over the past 31 years the carbon stocks have increased modestly,” Wang said. “What we estimate is that 430 million metric tons of biomass has accumulated over the last 31 years – but across this domain it would’ve been nearly double if it were not for these fires and harvests that are keeping it down.”

The assumption before, Wang explained, was that greening was happening and it was going to help draw climate-warming carbon dioxide concentrations down – but no-one knew the exact extent of that help.

To test the assumption, Wang and his team combined observational data from two different satellite missions from the US Geological Survey and NASA, Landsat and ICESat, so they could model the amount of carbon stored in biomass across a 2.8-million-square-kilometer region spanning Canada and Alaska.

ICESat data provides measurements of the height of forest canopies, while Landsat data extends back 31 years to 1984 and provides data on the reflection of different wavelengths of light from the surface of the planet, which also provides information about plant biomass abundance. Juxtaposing that with a two-to-three times increase in the severity of wildfires in the region, and the pictures started to take shape.

Wang found that plant biomass still increased, but not as much as previous computer models that aim to simulate climate change suggested they would, as those models have struggled to account for fires as a variable. The results, Wang hopes, will help scientists who construct those models – models that tell the world what we can expect climate change to look like – build ever-more-accurate pictures of what is in store as the century unfolds.

Co-author James Randerson from UC Irvine believes these new data are important because they provide an independent means to test climate models, and because of the way they represent feedbacks between the carbon cycle and the climate system. “The rates of carbon accumulation in this region are lower that what previous studies have indicated, and will push the science community to look elsewhere for the main drivers of the terrestrial carbon sink,” Randerson said.

Wang added: “The change is good news for climate — but it’s also much lower than we might’ve expected, because these fires have raged, and gotten more severe.”

Catastrophic sea-level rise from Antarctic melting possible with severe global warming

The Antarctic ice sheet is much less likely to become unstable and cause dramatic sea-level rise in upcoming centuries if the world follows policies that keep global warming below a key 2015 Paris climate agreement target, says a Rutgers coauthored study (DeConto, R.M., Pollard, D., Alley, R.B. et al, The Paris Climate Agreement and future sea-level rise from Antarctica, Nature, 2021 DOI: 10.1038/s41586-021-03427-0).

But if global warming exceeds the target – 2 degrees Celsius – the risk of ice shelves around the ice sheet’s perimeter melting would increase significantly, and their collapse would trigger rapid Antarctic melting. That would result in at least 0.07 inches of global average sea-level rise a year in 2060 and beyond, according to the study in the journal Nature.

That is faster than the average rate of sea-level rise over the past 120 years and, in vulnerable coastal places like downtown Annapolis, Maryland, has led to a dramatic increase in days of extreme flooding.

Global warming of 3 degrees Celsius could lead to catastrophic sea-level rise from Antarctic melting – an increase of at least 0.2 inches per year globally after 2060, on average.

“Ice-sheet collapse is irreversible over thousands of years, and if the Antarctic ice sheet becomes unstable it could continue to retreat for centuries,” said coauthor Daniel M. Gilford, a post-doctoral associate in the Rutgers Earth System Science & Policy Lab led by coauthor Robert E. Kopp, a professor in the Department of Earth and Planetary Sciences within the School of Arts and Sciences at Rutgers University-New Brunswick. “That’s regardless of whether emissions mitigation strategies such as removing carbon dioxide from the atmosphere are employed.”

The Paris Agreement, achieved at UN climate change conference, seeks to limit the negative impacts of global warming. Its goal is to keep the increase in global average temperature well below 2 degrees Celsius above pre-industrial levels, along with pursuing efforts to limit the increase to 1.5 degrees Celsius. The signatories committed to eliminating global net carbon dioxide emissions in the second half of the 21st century.

Climate change from human activities is causing sea levels to rise, and projecting how Antarctica will contribute to this rise in a warmer climate is a difficult but critical challenge. How ice sheets might respond to warming is not well understood, and we don’t know what the ultimate global policy response to climate change will be. Greenland is losing ice at a faster rate than Antarctica, but Antarctica contains nearly eight times more ice above the ocean level, equivalent to 190 feet of global average sea-level rise, the study notes.

The study explored how Antarctica might change over the next century and beyond, depending on whether the temperature targets in the Paris Agreement are met or exceeded. To better understand how the ice sheet might respond, scientists trained a state-of-the-art ice-sheet model with modern satellite observations, paleoclimate data and a machine learning technique. They used the model to explore the likelihood of rapid ice-sheet retreat and the western Antarctic ice-sheet’s collapse under different global greenhouse gas emissions policies.

Current international policies are likely to lead to about 3 degrees Celsius of warming, which could thin Antarctica’s protective ice shelves and trigger rapid ice-sheet retreat between 2050 and 2100. Under this scenario, geoengineering strategies such as removing carbon dioxide from the atmosphere and sequestering (or storing) it would fail to prevent the worst of Antarctica’s contributions to global sea-level rise.

“These results demonstrate the possibility that unstoppable, catastrophic sea level rise from Antarctica will be triggered if Paris Agreement temperature targets are exceeded,” the study says.

Gilford said, “It is critical to be proactive in mitigating climate change now through active international participation in reducing greenhouse gas emissions and by continuing to ratchet down proposed policies to meet the ambitious Paris Agreement targets.”



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