New observations and new research have increased our understanding of past, current, and future climate change since the Third U.S. National Climate Assessment (NCA3) was published in May 2014….
Since NCA3, stronger evidence has emerged for continuing, rapid, human-caused global warming of the global atmosphere and ocean. This report concludes that “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. For warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”
The last few years have also seen record-breaking, climate-related weather extremes, the three warmest years on record for the globe, and continued decline in arctic sea ice. These trends are expected to continue in the future over climate (multidecadal) timescales….
Global and U.S. Temperatures Continue to Rise
Recent data adds to the weight of evidence for rapid global-scale warming, the dominance of human causes, and the expected continuation of increasing temperatures, including more record-setting extremes….
Since the last National Climate Assessment was published, 2014 became the warmest year on record globally; 2015 surpassed 2014 by a wide margin; and 2016 surpassed 2015. Sixteen of the last 17 years are the warmest years on record for the globe.
… Longer-term climate records over past centuries and millennia indicate that average temperatures in recent decades over much of the world have been much higher, and have risen faster during this time period, than at any time in the past 1,700 years or more, the time period for which the global distribution of surface temperatures can be reconstructed.
Many Temperature and Precipitation Extremes Are Becoming More Common
Some extremes have already become more frequent, intense, or of longer duration, and many extremes are expected to increase or worsen, presenting substantial challenges for built, agricultural, and natural systems. Some storm types such as hurricanes, tornadoes, and winter storms are also exhibiting changes that have been linked to climate change, although the current state of the science does not yet permit detailed understanding.
There have been marked changes in temperature extremes across the contiguous United States. The number of high temperature records set in the past two decades far exceeds the number of low temperature records.
The Connected Climate System: Distant Changes Affect the United States
Understanding the full scope of human impacts on climate requires a global focus because of the interconnected nature of the climate system. For example, the climate of the Arctic and the climate of the continental United States are connected through atmospheric circulation patterns. While the Arctic may seem remote to most Americans, the climate effects of perturbations to arctic sea ice, land ice, surface temperature, snow cover, and permafrost affect the amount of warming, sea level change, carbon cycle impacts, and potentially even weather patterns in the lower 48 states. The Arctic is warming at a rate approximately twice as fast as the global average and, if it continues to warm at the same rate, Septembers will be nearly ice-free in the Arctic Ocean sometime between now and the 2040s.
Oceans Are Rising, Warming, and Becoming More Acidic
The world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century, making them warmer and altering global and regional climate feedbacks. Global mean sea level has risen by about 7–9 inches (about 16–21 cm) since 1900, with about 3 of those inches (about 7 cm) occurring since 1993.
Human-caused climate change has made a substantial contribution to global mean sea level rise since 1900, contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years. As sea levels have risen, the number of tidal floods each year that cause minor impacts (also called “nuisance floods”) have increased 5- to 10-fold since the 1960s in several U.S. coastal cities. Rates of increase are accelerating in over 25 Atlantic and Gulf Coast cities. Tidal flooding will continue increasing in depth, frequency, and extent this century.
Global Change in Alaska and across the Arctic Continues to Outpace Global Climate Change
Annual average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years at a rate more than twice as fast as the global average temperature…. It is virtually certain that human activities have contributed to Arctic surface temperature warming, sea ice loss since 1979, glacier mass loss, and northern hemisphere snow extent decline across the Arctic.
Limiting Globally Averaged Warming to 2 degrees C (3.6 F) Will Require Major Reductions in Emissions
Global mean atmospheric carbon dioxide (CO2) concentration has now passed 400 ppm, a level that last occurred 3 million years ago, when global average temperature and sea level were significantly higher than today. Continued growth in CO2 emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens of millions of years. The present-day emissions rate of nearly 10 GtC per year suggests that there is no climate analog to this century any time in at least the last 40 million years.
There is a Significant Possibility of Unanticipated Changes
Humanity is conducting an unprecedented experiment with the Earth’s climate system through emissions from large-scale fossil-fuel combustion, widespread deforestation, and other changes to the atmosphere and landscape…. There is significant potential for humankind’s planetary experiment to result in unanticipated surprises—and the further and faster the Earth’s climate system is changed, the greater the risk of such surprises.
There are at least two types of potential surprises: compound events, where multiple extreme climate events occur simultaneously or sequentially (creating greater overall impact), and critical threshold or tipping point events, where some threshold is crossed in the climate system (that leads to large impacts). The probability of such surprises—some of which may be abrupt and/or irreversible—as well as other more predictable but difficulty-to-manage impacts, increases as the influence of human activities on the climate system increases.
- USGCRP. 2017. Climate Science Special Report: A Sustained Assessment Activity of the U.S. Global Change Research Program. U.S. Global Change Research Program, Washington, D.C. USA. 669 pp. Available online at https://assets.documentcloud.org/documents/3920195/Final-Draft-of-the-Climate-Science-Special-Report.pdf
- The thirteen agencies include NSF, NASA, NOAA, EPA, the Smithsonian, and the de partments of Agriculture, Commerce, Energy, Defense, Health and Human Services, Interior, State, and Transportation.