The end of 2015 was filled with climate change policy talk, as the Paris conference brought a rare ray of hope, driven in part by a productive year of peer-reviewed climate science research.

How consequential were these latest scientific studies? Only time will tell, as citation patterns build up over time. But important research unveils changes and projections and opens the way for new understandings from across the globe involving seas and skies and glaciers and lakes, and ranging from drought-ridden California to war-ravaged Syria.

Consider some of the most important studies published in 2015, now setting the tone for research insights coming out this year:

Measurement Matters

There for years now has been a running public debate over whether or not available data indicate a “hiatus” or “pause” for rising temperatures and global warming over the past 17 years or so.

The UN’s own Intergovernmental Panel on Climate Change had, to some degree, acknowledged an apparent slowdown in their Fifth Assessment Report published in 2013. However, a major new paper published in Science in June by researchers at the National Oceanic and Atmospheric Administration (NOAA) indicated there has been no such pause, and new data correct the record. Lead author Thomas R. Karl and his co-authors stated that “there is no discernable (statistical or otherwise) decrease in the rate of warming between the second half of the 20th century and the first 15 years of the 21st century.” That means that the IPCC’s observation is, they write, “no longer valid.” The paper, amounting to a substantial challenge to those who maintain climate science “skepticism,” has even spawned a controversial Congressional investigation. (The paper doesn’t address what seems to an 18-year “pause” in heating of the troposphere, the lowest six or so miles of the atmosphere. But that trend is heavily influenced by natural variability, particularly the 1997-1998 El Niño that saw lower tropospheric temperatures spike upward by about 0.6 degrees C (1.1 degrees F).

Even as mega-trends were the subject of significant research, so too were more technical but extremely important questions, such as how precisely carbon dioxide changes the surface energy balance and warms Earth. We know about the greenhouse effect theoretically – and what computer models say – but can we directly measure the actual effect of CO2?

Yes, we can, according to an important March paper published in Nature. Lead author D.R. Feldman, of Lawrence Berkeley National Laboratory, and a research team analyzed weather data collected over more than a decade in Oklahoma and Alaska. They found that more heat radiation was striking Earth’s surface because the amount of CO2 in the atmosphere is increasing, and that the rate of increase accords with the results of climate models. They concluded that the empirical field data indeed “confirm theoretical predictions of the atmospheric greenhouse effect due to anthropogenic emissions….” (See more on this study.)

Changing Oceans

Climate change is already disrupting the delicate balance of the planet’s oceans, and one of the major effects is a decrease in oxygen levels. A new study published in March in Proceedings of the National Academy of Sciences (PNAS) provided a comprehensive look at how this de-oxygenation process may affect entire ecosystems.

Lead author Sarah E. Moffitt, of the University of California, Davis, and other researchers looked for clues in ocean sediment and fossil patterns relating to the last period of major warming – from about 16,000 to 3,400 years ago. Based on what happened in the past, the researchers conclude that the oceans will experience substantial disruption, as seafloor diversity shows a high degree of sensitivity to such changes: They find that “[F]uture anthropogenic climate change may significantly restructure the distribution of marine life on continental margins and that this disturbance can commit upper ocean ecosystems to millennia of ecological recovery.”

Changes in temperature also are affecting the complex natural circulation of ocean waters, with implications for weather patterns, ice masses, and much more. Researchers in this area continued in 2015 to show the complicated interconnections between temperature changes and the seas.

For example, a study published in February in Nature Climate Change analyzed trends in the Atlantic Ocean to get a better grasp on what exactly is happening with its major circulation flow in the North Atlantic, which appears to be weakening – an “unprecedented event in the past millennium.” Part of this trend, the researchers find, can be explained by the melting of Greenland’s ice mass, contributing to a large-scale freshening of ocean water. Uncertainties remain, the scientists noted, but over the next two decades we might expect to see even more circulation weakening, with big implications for the Labrador Sea.

Science of the Skies

Much of the most crucial atmospheric science research, while arcane for the public, points to critical new developments and understandings. A May 2015 study published in the Journal of Geophysical Research, for instance, analyzed the balance of energy coming into, and leaving, the planet and the role of water vapor and clouds. Lead author Kevin E. Trenbreth, of the National Center for Atmospheric Research, and other scientists made some fundamental insights into how to better measure the dynamics of climate change – namely that looking at temperatures in the troposphere, as opposed to surface temperatures, shows more precisely the effects of clouds and water vapor. It’s thought that clouds likely have an overall warming (“positive”) feedback, which this research confirmed. It also found a positive water vapor feedback, in accord with findings of the last decade. Figuring out what changes to clouds would mean is vital to understanding climate change more generally.

The relationship between extreme weather and changes in the atmosphere remains an area of intensive study – and continuing mystery. But a significant paper published in Nature in June helped clarify a bit more of the science in this regard, at least as it relates to changes in the circulation of the atmosphere.  A team led by Stanford University researchers in this study found that the “risk of extreme temperatures over some regions has also been altered by recent changes in the frequency, persistence, and maximum duration of regional circulation patterns,” although thermodynamic changes both regionally and globally – changes in the system’s heat content – account for a lot of extreme temperature. Their quantitative method, they state, “offers the potential to fingerprint dynamic and thermodynamic climate influences in isolation, which in turn may facilitate attribution of the observed trends, and projection of future trends.”

Continental Effects

Beyond the sea and skies, systematic and large-scale consequences of human-induced climate change are also increasingly being measured on land.

One of the most important scientific 2015 findings is that semi-arid zones seem to be expanding around the Earth, according to a blockbuster study published in Scientific Reports in August. Nanjing University researchers Duo Chan and Qigang Wu concluded that, over the past 60 years, warmer zones can be observed creeping everywhere around the planet:

“About 5.7 percent of the global total land area has shifted toward warmer and drier climate types from 1950–2010,” the authors wrote, “and significant changes include expansion of arid and high-latitude continental climate zones, shrinkage in polar and mid-latitude continental climates, poleward shifts in temperate, continental and polar climates, and increasing average elevation of tropical and polar climates.”

It is not just the land itself that appears to be changing, but also fresh-water sources. A major study published in Geophysical Research Letters in December makes significant observations based on analysis of 235 lakes across the world, representing more than half of all freshwater on the planet. First, the summer surface temperatures of these lakes are “warming significantly, with a mean trend of 0.34 degrees C per decade (0.61 degrees F per decade).” Second, however, there is substantial variation even among regionally neighboring lakes because of complex interactions between temperature and geomorphic features. As a result, the authors write, “one cannot assume that any individual lake has warmed concurrently with air temperature, for example, or that all lakes in a region are warming similarly.”

Ice and Glaciers

The melting of the glaciers and other ice sheets continues to command popular attention, but the inner-dynamics of change – and measurements of precise shifts – are still being investigated. Two studies in 2015 put forth major insights on these questions.

A study published in Earth and Planetary Sciences Letters in February models scenarios in which the Antarctic Ice Sheet experiences significant melting, adding into the model two new mechanisms that could accelerate change: “hydrofracturing due to increased surface melt,” and “structural failure of vertical ice cliffs.” Penn State’s David Pollard and Richard Alley, and Robert M. DeConto of University of Massachusetts, Amherst, noted in this study that the collapse of the West Antarctic Ice Sheet could happen within decades and, within an estimated several hundred to a few thousand years, the East Antarctic basins would recede, raising sea levels by some 17 meters, nearly 56 feet.

On the other side of the world the glaciers of Greenland, of course, are also in peril – and perhaps more than previously, authors of a new study in Geophysical Research Letters in July pointed out. Using echo sounding observations to study the underwater faces of Greenland glaciers, lead author Eric Rignot of the University of California, Irvine, and other scientists found that there are more cavities and more “undercutting” – diminishing ice mass stability and likely leading to more calving – than previously thought. The researchers noted the need for better empirical observations and more accurate models, as the “sensitivity of Greenland glaciers to ocean warming and enhanced ice sheet runoff may have been underestimated and projections of sea level rise from the Greenland Ice Sheet will need to be revised upward.”

Middle East Peril

Two high-profile studies published in the year just ended pertained to a region that has, tragically, long generated negative headlines for violent events: the Middle East.

And the climate science on that region also isn’t telling a happy story. A frightening but important paper published in Nature Climate Change in October used model simulations to predict end-of-century regional “wet bulb” temperature levels – a measure of how well one can cool their skin by sweating; even healthy people can’t tolerate a web-bulb temperature of 35 degrees C (95 degrees F), because it becomes impossible to lose heat by sweating – under a “business-as-usual” scenario of increasing greenhouse gas emissions. The projections suggest levels exceeding the 35 degrees C threshold for human survivability in the Arabian Gulf and elsewhere. Authors Jeremy S. Pal of Loyola Marymount/MIT and Elfatih A. B. Eltahirwrite of MIT noted that a “plausible analogy of future climate for many locations in Southwest Asia is the current climate of the desert of Northern Afar on the African side of the Red Sea, a region with no permanent human settlements owing to its extreme climate.” High wet-bulb temperatures were a deadly factor in India’s heat wave last summer which killed about 2,500 people.

Similarly, a new study published in PNAS in March showed how human-induced climate change likely contributed to Syria’s historic drought, leading to mass migrations to urban areas and setting the table for unrest that added fueled fuel to the Syrian Civil War. Researchers of that paper, by lead author Colin P. Kelley of UC-Santa Barbara and others at Columbia University, found that the drought seen in Syria, which began in the mid-2000s, was made “twice as likely as a consequence of human interference in the climate system.”

Homeland Insecurity

Impacts of climate change on the United States are studied across all regions, but the southwestern portion of the country in particular has continued to be an area of focus because of historic drought conditions. Many studies over the past year add to our understanding of what is going on, from California to the Central Plains.

For example, authors of a February paper in Science Advances used climate models to compare future warming scenarios in those regions to historic patterns of drought. How do projected droughts stack-up against past droughts not caused by human activity? Benjamin I. Cook, of NASA Goddard Institute for Space Studies, and co-authors found that the “mean state of drought in the late 21st century over the Central Plains and Southwest will likely exceed even the most severe megadrought periods of the Medieval era (which created the Sand Hill dunes in Nebraska).” That is the case, they wrote, “under both high and moderate future emissions scenarios, representing an unprecedented fundamental climate shift with respect to the last millennium.” (A related paper published in Geophysical Research Letters also found that the California drought of 2012-2014 was the most severe over the past 1,200 years.)

No compilation of a year’s outstanding climate science research may ever be one-hundred percent “complete” and beyond being subject to challenge. Others no doubt could and will have their own candidate “favorites” for inclusion, beyond those cited here. While there likely will be fewer challenges to the underlying significance of the individual research studies mentioned above, Yale Climate Connections welcomes scientists’ and others’ recommendations of additional major studies deserving recognition in such a listing.