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 October and November 2019.

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  • North American bird populations decline significantly: Scientists found that bird populations dropped by nearly 30% — almost 3 billion birds — since 1970. The birds include rare as well as abundant species. Authors note that threats to birds, such as agricultural intensification and habitat loss, are all exacerbated by climate change.
  • More Ebola: Modelers estimated that the risk of Ebola spreading from animals to humans will increase in Africa by a factor of 1.75 to 3.2 by 2070. Disease spread will be greatest in areas facing high warming and population growth and low economic development.
  • Sea level rise threatens more people than previously thought: Researchers found that the number of people vulnerable to sea level rise is triple the previous estimate. Under a low-emissions scenario, high tides will inundate areas where 190 million people live by the end of the century. That number increases to up to 630 million under a high-emissions scenario.
  • Arctic sea ice decline fuels disease: The phocine distemper virus (PDV) has caused extensive mortality in Atlantic seals in the North Atlantic. The disease is now spreading to other seal species and sea otters, as well as across the Atlantic to the Pacific. Scientists surmise that because of reductions in Arctic sea ice extent, marine mammals are in greater contact, allowing for more disease transmission.
  • Drought triggered Assyrian Empire collapse:  Researchers traced the rise of the Assyrian Empire to a time marked by centuries-long wet climate. They further found that a megadrought caused a decline in agricultural productivity, with in turn led to political unrest, the civilization’s economic downfall, and its eventual collapse.
  • Tipping points dangerously close: A tipping point is a threshold that, when reached, causes significant changes in the Earth’s system — like the disintegration of the West Antarctic ice sheet, which would cause catastrophic sea level rise, or loss of Amazon rainforest, which would cause runaway warming and the loss of countless species. Tipping points have catastrophic, irreversible consequences. A new study found that several tipping points in the climate system are in close reach, and that some may be exceeded even if global temperature rise is limited to between 1 degrees C (1.8 degrees F) and 2 degrees C (3.6 degrees F).
  • Penguins march toward extinction: Scientists found that emperor penguin populations will decline by at least 80% by 2100 if warming continues unabated. Populations will still decline at least 31% if global temperature rise is limited to 1.5 degrees C, and 44% at 2 degrees C.
  • Warming weakens U.S. bridges: Researchers found that warming exacerbates the malfunctioning of bridges in the United States, which are already deteriorating, as they accumulate debris and dirt in their joints.
  • Marine species at particular risk: Assessing 239 studies, scientists found that species composition change due to climate change and other stressors has been widespread. The marine tropics have experienced the greatest loss in species richness, especially in the western Atlantic and northwest Australia.


  • The emissions gap is huge: The 2019 UNEP Emissions Gap Report found that emissions need to be slashed in half by 2030 and reduced 7.6% a year over the next decade to have a good chance of limiting warming to 1.5 degrees C, the limit necessary for averting the worst impacts of climate change.
  • Arctic permafrost emitting more than it absorbs: Warming of permafrost increases microbes’ decomposition in soils, in turn releasing carbon dioxide. Scientists found that the amount of carbon dioxide permafrost is losing during the winter months is now greater than the amount it absorbs. Under a moderate emissions-reduction scenario, winter carbon dioxide emissions from permafrost will increase by 17% by the end of the century, and by 41% if warming continues unabated.
  • Emissions from deforestation of intact forests higher than expected: Previous estimates of carbon emissions from deforestation across the tropics had not considered factors such as selective logging, forest fragmentation, and forests’ loss of carbon sequestration abilities. When these factors are included, the carbon impact from the loss of intact tropical forests increases by a factor of 6. Accordingly, the climate benefit associated with protecting intact tropical forests, which house tremendous biodiversity, is much larger than previously assumed.
  • Nitrous oxide on the rise: Using new estimation methods, scientists found that nitrous oxide, a greenhouse gas and ozone-depleting substance, increased significantly after 2009, especially in East Asia and South America. Researchers concluded that traditional estimation methodologies underestimated the increased rate of emissions.
  • Hydropower’s climate benefit varies: Constructing hydropower requires flooding ecosystems, which can affect carbon cycling and lead the landscape to become a carbon source, rather than a sink, as vegetation is submerged. One study found that while hydropower is less emissions-intensive than coal and natural gas overall, certain sites have greater climate impacts. Hydropower results in more emissions than natural gas-generated electricity in Western Africa and parts of South America, and more emissions than coal at most facilities in Western Africa.

Extreme events

  • El Niño to become more extreme: Researchers studied how El Niño events, which can cause severe droughts and floods, have changed from 1901 to 2017. They found that since the 1980s, strong El Niño events coinciding with warming have increased and are expected to cause “profound socioeconomic consequences” in the future.
  • Weather gets “stuck” in Northern Hemisphere: Atmospheric blocking events are high-pressure systems that persist for days to weeks, which can lead to prolonged extreme events. Scientists found that the area covered by these blocking events will increase by up to 17% in a high-emissions scenario, especially in the Northern Hemisphere.
  • New connection between hurricane damages and climate change: While it is well known that economic damages associated with hurricanes have been increasing in recent years, it has been difficult to understand how much of those damages are associated with increased wealth exposure — for example, more people living in vulnerable areas — versus human-induced climate change. Researchers found a new technique to “normalize” costs in order to make events comparable, accounting for greater exposure over time. When scientists normalized the costs of hurricane damage in the United States from 1900-2018, it was still evident that hurricanes have become much more damaging in a warming climate.


  • Glaciers in Papua to disappear: Scientists projected that glaciers in Papua, Indonesia will likely disappear in the next decade. These are the only remaining tropical glaciers between the Andes and Himalayas.
  • Draining glacial lakes: Studying drainage dynamics of a Greenland Ice Sheet lake, scientists posited that rapid lake drainages from ice fractures are more extensive than previously thought. This drainage can in turn lubricate the ice sheet and lead to faster ice flow, resulting in higher levels of sea level rise.
  • Glacial melt reveals Arctic islands: The Russian navy announced that it found five new islands in the Arctic that had previously been covered by glaciers.