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 December 2019. (To get these updates delivered right to your inbox, sign up for our newsletter.)


  • Earlier baby deliveries: A study found that extreme heat in the United States led pregnant women to deliver their babies before their due dates. Between 1969 and 1988, around 25,000 babies were born earlier each year due to heat exposure. Health and cognitive challenges have been linked to pre-term births.
  • Shrinking birds: Scientists assessed 52 migratory bird species across North America over a 40-year period. They found that the birds' body sizes decreased with warming, while wing length increased. They hypothesize that the longer wing length assists smaller birds in maintaining migration.
  • Shifting migrations: A study collected data on overnight bird migration in the United States and found that warming led to earlier arrivals of species. This is problematic because migration is typically timed with resource availability; a mismatch can threaten populations and have broader implications for ecosystem structure and function.
  • Stranded turtles: Kemp ridley turtles are traveling farther north in the Atlantic due to warmer waters and then getting "stuck" in Cape Cod's hooked peninsula. When winter comes, they are "cold-stunned," leading to strandings . Turtles are very sensitive to temperature change; temperature drops can stress and potentially kill them.
  • Parrotfish withstand coral bleaching: In a study of reefs in two ocean basins, scientists found that parrotfish populations grew after bleaching-related coral deaths. This was in sharp contrast to almost every other fish species, which declined after coral bleachings. Scientists noted that bleaching creates newly barren surfaces that get colonized by microalgae and cyanobacteria, the parrotfish's food source, but that parrotfish numbers would decline as the reef returned to health.
  • Fewer New England fishermen: About 34,000 fishermen are employed in New England, where waters are warming quickly. One study found that county-level fishing employment in New England declined by 16% on average between 1996 and 2017, likely due to changes in climate variability.
  • Heat-stressed mountain goats: Scientists studied the impacts of heat on mountain goats in Glacier National Park, Montana. Snow patches ordinarily provide a refuge for the species to reduce heat stress. But given reductions in persistent summer snow, animals may not be able to find areas for resting and reducing their breathing rates in the heat.
  • Changing water supplies: Mountain ranges store water and supply it downstream, much like a natural water tower. One study identified the world's water-tower ranges and found that the most important ones are also the most vulnerable. Given climatic and socio-economic changes in the future, 1.9 billion people living in or downstream of mountain areas could be negatively impacted.

Extreme Events

  • Simultaneous heat extremes: Researchers reported that two particular patterns of the jet stream can lead to heat extremes occurring in multiple parts of the world simultaneously. These patterns can reduce regional crop production by up to 11%. If multiple regions are affected at the same time, the consequences for global food production and security could be grave. The authors noted that some studies have found that such jet stream patterns have increased in recent summers.


  • Greenland ice loss speeding up: Researchers analyzed measurements from 26 satellites to assess changes in Greenland's ice sheet. They found Greenland lost about 3,800 gigatonnes (Gt) of ice between 1992 and 2018, contributing to almost 11 millimeters (0.43 inches) of sea level rise. The rate of ice loss increased over this period, from around 18 Gt/year between 1992 and 1997 to 239 Gt/year between 2012 and 2017. Scientists noted that cumulative ice loss tracks with the high end of the IPCC's climate-warming scenario.
  • Greenland lakes draining faster than we thought: Scientists found that a West Greenland lake lost two-thirds of its volume in just five hours in 2018 and was drained only one year earlier. Partial drainage events were previously thought to occur slowly, but scientists found that hydrofractures, in which surface melt reaches the base of the ice sheet via water-driven cracks in the ice, can lead to much more rapid drainage. In this case, the lake expanded and reactivated a preexisting fracture.
  • A transformed Arctic and Antarctic: Warming is greatest at the poles. For example, with 2 degrees C of global warming, the Arctic may see 4 degrees C of average annual warming and 7 degrees C of winter warming. A new study looked at the consequence of this extreme warming in the Arctic and Antarctic. In the Arctic, scientists found that warming will lead to ice and wildlife losses, threats to livelihoods, extreme weather at lower latitudes, and increased emissions as permafrost thaws. Collapse of the Thwaites Glacier and other Antarctic ice could lead to 3 meters (9 ft) of global sea level rise, possibly in less than a century.
  • Arctic warming accelerating: The U.S. National Oceanic and Atmospheric Administration's 2019 Arctic Report Card documented unprecedented changes in the region – including temperature rise, ice sheet and snow cover loss, thawing permafrost and smaller sea ice extent. Changes are occurring more quickly than anticipated, and ecosystems and communities are at increased risk.
  • Loss of snow leads to greater warming: When snow cover melts, the surface it was previously covering becomes darker, absorbing more solar radiation and triggering greater warming. Scientists found that declining snow in North America can cause up to 2 degrees C (3.6 degrees F) of warming annually in the region and 4 degrees C (7.2 degrees F) in the spring. It can also intensify warm extremes. This effect is greatest in northern Canada, given its abundant snow cover.
  • Ice loss in Russian Arctic discovered: Researchers discovered rapid melting and a new ice stream on the Vavilov Ice Cap in the Russian Arctic. It has lost more than 11% of the entire basin's ice mass between 2013 and 2019. Authors documented a speeding up of glacial flow, in which ice quickly drains from the glacier and flows into the ocean, creating an "ice stream." The study is the first documented case of the formation of an ice stream.


  • Record-high carbon dioxide emissions from fossil fuels: The Global Carbon Project found that carbon dioxide emissions from fossil fuels were on track to reach a record high in 2019.
  • Underestimated methane emissions: Using satellite data, researchers found that methane emissions associated with a 20-day natural gas well blowout in Ohio in 2018 were far greater than what was reported. Methane emissions were more than the emissions associated with oil and gas industries in France and Norway. The authors recommend that such satellite measurements be used in detecting and quantifying emissions from other events.
  • Tropical Africa a major methane source: Methane is a potent greenhouse gas, about 28 times more powerful than carbon dioxide. Scientists discovered that between 2010 and 2016, methane emissions from tropical Africa increased substantially, representing a third of global methane emissions growth during this period. This may be due to emissions associated with agriculture and wetlands.
  • Nitrous oxide-emitting rivers: Scientists found that nitrous oxide emissions from rivers jumped from 70.4 gigagrams (Gg)/year to 291.3 Gg/year between 1900 and 2016. Nitrous oxide emissions from rivers occur mainly due to fertilizer use in agriculture. While riverine N20 emissions are only 3% of global terrestrial N2O emissions, they are increasing 3 times faster than other sources.
  • Ozone hole recovery could be delayed: A new modeling effort found that the hole in the ozone would close by 2065 if countries comply with the Montreal Protocol. However, if CFC-11 emissions increase, as they have been recently, recovery could be delayed by more than a decade.


  • 2019 rounds out warmest decade on record: The World Meteorological Organization stated that 2019 was on course to be the second- or third-warmest year on record, capping off the warmest decade on record. The global average temperature for January to October 2019 was 1.1 degrees C higher than the pre-industrial era.
  • A warm 2020: The UK Met Office released its forecast for 2020 temperature, estimating that it will be about 1.11 degrees C over the pre-industrial average. The warmest year on record, 2016, was 1.16 degrees C over the pre-industrial level.
  • Scientists say models are accurate: In an effort to understand the accuracy of climate models, scientists compared model projections of global mean surface temperature to observations between 1970 and 2007. They found that climate modeling efforts over the past 50 years were accurate in predicting changes in global mean surface temperature, which should boost confidence in models' ability to project global warming.

The Ocean

  • California seas more acidic than others: The ocean is becoming more acidic as it absorbs carbon dioxide. Scientists used almost 2,000 measurements to piece together an ocean acidification record for waters off the coast of California, finding they are 2 times more acidic than the global average. Marine organisms and the entire food web can be at risk when oceans become acidic.
  • Reduced oxygen levels: Climate models suggest that warming will lead to a decline in dissolved oxygen in oceans. Oxygen-poor waters can result in profound changes to marine ecosystems, as some organisms avoid or cannot survive in such areas. Scientists collected data over the past 50 years in waters of the eastern tropical Atlantic and equatorial Pacific and found that oxygen has already declined 0.09-0.34 micromoles per kilogram per year.