This Month in Climate Science: Carbon Sinks Shrink, Bumble Bees Decline, Nightingale Wings Shorten, and Ecosystems Could Collapse Faster

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Originally published on WRI’s Resource Institute Blog.
by Kelly Levin and Dennis Tirpak

Every month, climate scientists make new discoveries that advance our understanding of climate change’s causes and impacts. The research gives a clearer picture of the threats we already face and explores what’s to come if we don’t reduce emissions at a quicker pace.

Our blog series, This Month in Climate Science, offers a snapshot of the month’s significant scientific literature, compiled from some of the leading peer-reviewed journals. This edition explores studies published in February and March 2020. (To get these updates delivered right to your inbox, sign up for our Hot Science newsletter.)

Recent Extreme Events

The past few months have brought alarming signs of a changing climate, many of which are in line with projections of a warming world:

• Record warmth in Antarctica: Antarctica witnessed its hottest day on record, reaching 18.3 degrees C (65 degrees F). In February, an iceberg almost the size of Atlanta broke off of Pine Glacier and 20% of the ice cap of Eagle Island, Antarctica, melted within days, covering it with melt ponds.
• Second warmest summer in Australia: This year, Australia also saw its second hottest summer on record, 1.9 degrees C (3.4 degrees F) above average.
• Australia also witnessing most widespread coral bleaching event on record:  Scientists are reporting that Australia is currently experiencing a new bleaching event that is the most widespread on record, the third major bleaching event in only five years. Researchers note that such events put coral reefs “closer to their upper thermal limits.”

• Hottest winter on record in EU and Russia: According to the EU’s Copernicus Climate Change Service, this past winter in the EU was the hottest on record: 3.4 degrees C (6.1 degrees F) above the 1981-2010 average. According to the Hydrometeorological Center of Russia, Russia also saw its hottest winter ever, 1.3 degrees C (2.3 degrees F) above the previous record winter of 2015-6. Moscow experienced a winter that was 7.5 degrees C (13.5 degrees F) above average.
• Low levels of snow in many locations: In Helsinki, January and February saw no snow. The East Coast of the U.S. saw well-below average snowfall. California had its driest February on record, and the snowpack was only 44% of its typical amount.
• Spring coming earlier in the U.S.: According to the U.S. National Phenology Network, the first leaves are coming three to four weeks earlier in spring than the average between 1981-2010 in portions of the Northeast, Southeast and Western United States.

Impacts

• Loss of carbon sink: Intact forests in the tropics sequestered roughly 15% of human-induced carbon dioxide emissions in the 1990s and early 2000s. Researchers examining carbon sinks across African tropical forests and across Amazonia found that the carbon sink in African tropical forests has been stable for three decades, while the Amazonian forests have seen a decline. Using models accounting for future warming, they predict  a future decline in the African sink and a rapid weakening of the Amazonian sink. After the 2030s, the Amazonian tropical forests will stop sequestering carbon, according to their analysis. They conclude that the uptake of carbon from intact tropical forests peaked in the 1990s; accordingly, the global community will need to reach net zero greenhouse gas (GHG) emissions even earlier to compensate for the loss of carbon sink.

• Collapse of large ecosystems can occur rapidly:Scientists analyzed data from marine, freshwater and terrestrial ecosystems and examined the time it takes for a system to collapse. They found that large vulnerable ecosystems, including the Amazon rainforest and coral reefs in the Caribbean, can collapse in just a few decades once they are triggered. They suggest that a tipping point may be reached in the Amazon as early as 2021, and the Caribbean coral cover could completely disappear by 2035.
• Widespread land degradation and desertification in our future: More than 40% of the Earth is covered in drylands and one-third of the global population resides there. A new study anticipates that 20% of the land surface will witness abrupt declines in plant productivity, plant cover and soil fertility. The authors call for immediate action to minimize the negative impacts, which will affect more than 2 billion people.
• Warmer extremes leading to decline of bumble bees: Bumble bee species have seen a widespread decline in recent years, with far-reaching implications given their critical role as pollinators. Collecting data for 66 bumble bee species across Europe and North America, researchers found a decline in the bees by 46% across North America and 17% in Europe relative to a baseline period of 1901-1974. They found that frequent warm temperatures is a predictor of species’ local extinction risk and their ability to colonize a new area. While there are other impacts to bumble bees, such as land use change and pesticide use, the scientists were able to isolate the impacts of climate change on extinction risk.
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• Extinction of species due to hot temperatures: Studying surveys of 538 plant and animal species – 44% of which already had local extinctions – scientists found that the places with the largest and fastest local extinctions were ones that experienced hottest yearly temperatures. Based on past rates of dispersal, the authors estimate that 57-70% of the 538 species will face extinction.  When the species are able to shift to different niches, this number could be lowered to 16-30% of species.

• Fewer polar bear cubs due to sea ice loss: Studying data from Baffin Bay, Canada, scientists found that polar bears spent an additional 30 days on land from 2009-2015 compared with 1991-1997, which they attribute to changes in spring sea ice breakup and autumn sea ice formation. With longer ice-free periods, they found polar bears are having smaller litters. Based on these findings, they project that as sea ice loss continues, two-cub litters could largely disappear, with profound implications for polar bear survival.
• Shrinking ice leading to hard choices for polar bears: In another study of polar bears in the Southern Beaufort Sea, researchers found that bears that choose to live on receding sea ice use less energy from May through September, even though they eventually need to expend energy on swimming once the sea ice recedes. In contrast, those bears that moved to land more quickly used more upfront energy to do so, but were able to spend more time in preferred hunting habitats and had access to whale carcasses. The authors suggest there could be a larger benefit to move to land and that diminishing summer sea ice functions as an “ecological trap.” The large majority (73-85%) of the subpopulation studied stayed on sea ice in the summer.
• Nightingale wings are getting shorter, compromising migration: A study of two common nightingale populations in Spain shows that wing length relative to body size has declined from 1995 to 2014, a period marked by increased summer drought which led to breeding declines. The shorter winged nightingales did not survive as well during migration, although they saw reproductive benefits.
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• More drought-tolerant tree communities: A study of US forests found that forest ecosystems have shifted to become more drought-tolerant since 2000. The shifts were the greatest in drier regions which had experienced significant tree mortality of trees that were less hardy to drought.
• Marine species shift poleward: Studying more than 300 widely distributed marine species, from phytoplankton to marine mammals, scientists found large-scale changes in the abundance of species, consistent with warming. Species abundance is increasing poleward and declining at the equator given species’ failure to adapt to rapid climate change. The authors conclude that these impacts could have profound effects on coastal economies that depend upon marine species.

• Lakes to become warmer:Researchers found that by the end of the century, under a high emissions scenario, almost two-thirds of lakes would change to a type of lake that is more typical of lower latitudes with warmer surface water temperatures (vs. 12% under a low emissions scenario). In a worst-case scenario, almost 80% of northern frigid lakes would be transformed. This could have significant implications on species that depend on cold temperatures and ice cover.
• Slow recovery of tropical vegetation after El Niño event: In 2015-6, a significant El Niño event caused severe drought and extreme heat, which led to large carbon emissions from vegetation in the tropics. Researchers found that by the end of 2017, above-ground carbon stocks had not recovered to the pre-drought levels of 2014. The inability of forests to recover from drought can lead to long-term forest loss and compromise carbon storage.

• Disappearing beaches: More than one-third of the world’s coastline is covered by sandy beaches, which provide critical tourism, recreation and ecosystem services. A new study found that sea level rise, among other factors, could make almost half the world’s sandy beaches disappear by 2100. Moderate climate action could prevent 40% of this shoreline retreat. A large portion of the threatened shorelines are in densely populated areas, underscoring the additional need for adaptation. Australia, Canada, Chile, Mexico, China, the U.S., Russia and Argentina are among the countries most affected.
• Colorado River flow declining: A recent study found that the average discharge of the Colorado River has declined each year by 9.3% per degree C (1.8 degrees F) of warming. They attribute this to increased evapotranspiration (a process by which water is transferred from the land to the atmosphere) due to loss of snow and increase in absorption of solar radiation. By midcentury, the river’s flow could decline by as much as 31% compared to historical levels. The authors suggest a heightened risk of severe water shortages in the future.
• Air pollution causes more premature deaths than smoking:  Researchers found that air pollution leads to higher levels of death and loss of life expectancy than smoking. The risk posed by air pollution is also an order of magnitude higher than violence. If fossil fuel emissions were halted, the authors found that global average life expectancy would increase by more than a year. If all air pollution sources were eliminated, life expectancy would rise by almost two years.

Ice

• Overlooked risk of abrupt thaw of permafrost:Scientists synthesized the data on permafrost thaw and find that we are underestimating the risk of abrupt thaw. A region that is 2.5 million square kilometers (965,000 square miles) in size that is subject to abrupt thaw could be equivalent to gradual thaw of the entire 18 million square kilometer (nearly 7 million square miles) permafrost region under a high emissions scenario. Even if 5% of the Arctic permafrost abruptly thaws, that could double permafrost’s contribution to warming. Abrupt thaw is not included in Earth system models. Permafrost regions house about 60% of the world’s soil carbon and when they are thawed, they release carbon, further exacerbating warming.

• Close to a tipping point for multiple meters of sea level rise: Using multiple proxies to reconstruct a record of the West Antarctic Ice Sheet during the Last Interglacial (129,000 to 116,000 years ago), scientists revealed that Antarctica could have contributed multiple meters of global sea level at the time. This period was associated with warming of the oceans and release of methane from marine sediments. The researchers conclude that the ice sheet is currently “close to a tipping point” given current projections of warming.
• Retreating of large glacier in Antarctica: The Denman Glacier in East Antarctica holds an ice volume equivalent to 1.5 meters (nearly 5 feet) of sea level rise. Scientists found that the grounding line of the glacier, where the grounded part of the ice meets the floating part of the ice, retreated 5.4 km (3.4 miles) in the last 21 years. They note the potential for rapid retreat along a deep trough that had not previously been identified.

• Walls of Antarctic glaciers offer protection from warm seawater: Studying Gertz glacier in Antarctica, scientists found that the vertical face of the ice edge plays a critical role in limiting the transport of warm seawater deeper into the continent. If these ice walls were to disappear, the ice on land would be left much more vulnerable.
• Loss of Arctic and Antarctic ice impacts the tropics:Researchers found that Arctic and Antarctic sea ice loss will have far reaching impacts – all the way to the tropics – given ocean dynamics. Under a high emissions scenario, the authors project that the sea ice loss will contribute 20-30% of future warming and precipitation changes in the tropics. In another study, scientists found that Arctic sea ice loss has the potential to influence tropical Pacific sea surface temperature.

• Loss of South Georgia’s glaciers: Few studies have been conducted on changes to glaciers on Antarctic islands, such as South Georgia. Researchers using remote sensing found a significant mass loss of South Georgia’s glaciers beginning this century. Between 2003 and 2016, about 4% of the total glacier area was lost.
• Quantifying recent ice loss in Greenland: New satellite data reveals that the amount of ice lost from the Greenland Ice Sheet this past summer was almost 2.5 times that of the average annual loss from 2002 and 2019.
• Algae on Greenland Ice Sheet thriving, darkening and melting ice:Researchers found that blooms of algae on the surface of the Greenland Ice Sheet in summer have darkened and melted the ice. They note that this process will be critical to incorporate into models of ice sheet surface runoff to more accurately estimate future ice mass loss and contributions to sea level rise.
• Warm waters melting underside of Greenland’s ice: The loss of the Greenland ice sheet over the last 20 years has contributed about 25% of global sea level rise. Scientists have now advanced our understanding of how the Northeast Greenland ice stream is changing in a warmer world. The ice stream is the largest floating glacial tongue on the coast of Greenland and drains about 16% of the entire Greenland ice sheet. Studying data from 2016 and 2017, the scientists found an inflow of warm Atlantic water, melting the glacier from underwater.

Ocean Behavior

• Ocean circulation patterns not as stable as previously thought during warm periods: The Atlantic Meridional Overturning Circulation and North Atlantic Deep Water, two related ocean circulation processes, were previously thought to be stable during interglacial periods that were warm. A new study found that instability has been common and long-lasting during interglacials, and that future large North Atlantic Deep Water disruptions may be more likely than we had previously leading to very cold periods in Europe.
• North Atlantic “warming hole” attributed to emissions:  A portion of the North Atlantic – dubbed the “North Atlantic Warming Hole” — is expected to be considerably cooler than other regions of the ocean. This is surprising, given the expectation that sea surface temperatures rise with increasing greenhouse gas emissions. A new study found that human-induced emissions are leading to this warming hole as ocean circulation changes and warming is reduced at midlatitudes compared with higher latitudes. This pattern has implications for the climate of the U.S., Europe and parts of Africa.

• Ocean circulation speeding up: Examining data from the early 1990s, scientists found that average ocean circulation has been accelerating significantly. This is particularly evident in tropical oceans. The detected acceleration is much larger than what would have occurred with natural variability. Ocean circulation affects weather patterns, ocean heat storage, and the jet stream.
• Major current out of balance:Scientists found that the loss of Arctic sea ice is increasing eddy activity in the Beaufort Gyre, a major circular current. The gyre plays in an important role in keeping the polar environment in equilibrium. Given the increase in freshwater and energy from persistent winds, the delicate balance between the gyre and surrounding oceans could be compromised, with freshwater leaving the gyre at greater rates, entering the North Atlantic.

Featured photo © Kelly Levin/WRI