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U.S. Climate Assessment: Now Is the Warmest Period of Modern Times

A comprehensive new U.S. government report released today confirms the well-established science behind climate change: it is real, it is human-caused, it is happening faster than predicted and it poses a tremendous threat to America and the rest of the world. Titled the Climate Science Special Report, it is the first of two parts of the congressionally mandated National Climate Assessment, last published in 2014. It represents our best, most considered views of the reality of human-caused climate change. As with previous reports in this series, it does not provide policy advice, but it does indicate our most considered view of where we stand on climate change.

Report Highlights

  • Global annually averaged surface air temperature, and the annually averaged temperature in the U.S., has increased by about 1.8 degrees F (1 degree C) over the last 115 years (1901–2016), with Alaska warming twice as much. Last year was the third year in a row, following 2015 and 2014, to set a new global record for the warmest year. This is now the warmest period in the history of modern civilization.

  • Based on extensive evidence, there is no convincing alternative explanation supported by the extend of the observational evidence, that anything other than human activity is the dominant cause of the observed warming since the mid-20th century.

  • Global atmospheric carbon dioxide concentrations have now passed 400 parts per million (ppm), a level that last occurred about 3 million years ago, when both global average temperature and sea level were significantly higher than today. Continued growth in these emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens to hundreds of millions of years.

  • Human-caused climate change has made a substantial contribution to the observed 7-8 inches of global average sea level rise since 1900, a greater rate of rise in at least 2,800 years. Global average sea level is expected to continue to rise by at least several inches in the next 15 years and by 1–4 feet by 2100. A rise of as much as 8 feet by 2100 cannot be ruled out.  

What is New

The report notes that there are now many new sources confirming climate impacts, including the acceleration of ice sheet loss, and takes stock of new research and understanding on ocean acidification and warming, among other recent changes.

In addition, the report reflects significant advances in science since the last report in 2014. Projections of climate change now offer greater understanding of local impacts because models now have higher resolution, and sea level rise projections now better incorporate variations across different regions in the United States. In addition, global climate models can now more realistically simulate intense weather systems, including hurricanes, and are better able to project extreme weather with greater confidence.

There have also been scientific advances in in the detection and attribution of human activities in extreme climate and weather events. The report pays particular attention to extreme events in the U.S., where the science of event attribution has evolved significantly, especially in the aftermath of recent extreme events, for example, the recent California drought.

For the first time, the report includes a discussion of climate-related “surprises,” or unanticipated changes, in which tipping points in the Earth’s systems are crossed or climate-related extreme events happen at the same time, creating “compound extreme events,” multiplying the potential damage and destruction.

The extent of Arctic sea ice – the area of ocean water that ice covers around the North Pole – may change abruptly, becoming ice-free first in the summer and then perennially, the report said. Warming may lower the threshold that triggers extreme El Niño and La Niña events. There is also evidence of the slowing of and risk of a possible collapse of the Atlantic Ocean’s “conveyor belt” current – known to scientists as the Atlantic meridional overturning circulation, which carries warm water from the Caribbean northward toward the British Isles and northern Europe -- leading to significant sea level rise, especially along the northeastern U.S. coast. The ocean’s ability to absorb carbon dioxide could be reduced as a result. In addition, if permafrost melts, releasing its long-held carbon dioxide or methane into the atmosphere, and methane hydrates at the bottom of the continental shelves of the Arctic Ocean are destabilized, there could be highly accelerated warming. The report concludes that “climate models are more likely to underestimate than to overestimate the amount of long-term future change.”

A Sliver of Good News

In a somewhat hopeful note, the report found that in 2014 and 2015, carbon dioxide emissions growth rates slowed as economic growth became less carbon-intensive. However, even if this positive trend continues, it is not yet enough to limit global average temperature rise to well below 3.6 degrees F (2 degrees C) above pre-industrial levels, which is the internationally established climate stabilization goal as articulated in the Paris Agreement.

The second part of this year’s National Climate Assessment, Climate Change Impacts, Risks and Adaptation in the U.S., to be released next year, will detail the impacts climate change will have across multiple U.S. sectors, broken down into 10 discrete regions, including, for the first time, the Caribbean as its own region of analysis. This will provide the best picture yet of how climate change threatens specific American communities and offer an invaluable tool to leaders who want to protect their citizens. As we look forward to that report, we should re-double our efforts to address this daunting challenge.

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