Earlier this month the government issued its fourth climate science report (NCA4) which, among other things, reinforces the scientific evidence of the contribution by humans to climate change. It is important to note the Report is mandated by the Global Climate Assessment Act of 1990 and was prepared by a group of agencies and individuals with significant credentials:
“The National Oceanic and Atmospheric Administration (NOAA) serves as the administrative lead agency for the preparation of NCA4. The CSSR Federal Science Steering Committee (SSC)1 has representatives from three agencies (NOAA, the National Aeronautics and Space Administration [NASA], and the Department of Energy [DOE]); USGCRP; and three Coordinating Lead Authors, all of whom were Federal employees during the development of this report. Following a public notice for author nominations in March 2016, the SSC selected the writing team, consisting of scientists representing Federal agencies, national laboratories, universities, and the private sector. Contributing Authors were requested to provide special input to the Lead Authors to help with specific issues of the assessment.”
A sampling of the sobering conclusions of the Report regarding the increasing human influence on climate change, the already extreme impacts and the potential for even more severe effects are quoted below:
“Thousands of studies conducted by tens of thousands of scientists around the world have documented changes in surface, atmospheric, and oceanic temperatures; melting glaciers; disappearing snow cover; shrinking sea ice; rising sea level; and an increase in atmospheric water vapor. Rainfall patterns and storms are changing, and the occurrence of droughts is shifting….
The Intergovernmental Panel on Climate Change concluded that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. Over the last century, there are no alternative explanations supported by the evidence that are either credible or that can contribute more than marginally to the observed patterns. There is no convincing evidence that natural variability can account for the amount of and the pattern of global warming observed over the industrial era. …
Solar flux variations over the last six decades have been too small to explain the observed changes in climate. There are no apparent natural cycles in the observational record that can explain the recent changes in climate (e.g., PAGES 2k Consortium 2013; Marcott et al. 2013; Otto-Bliesner et al. 2016 ). In addition, natural cycles within Earth’s climate system can only redistribute heat; they cannot be responsible for the observed increase in the overall heat content of the climate system….
Globally, the persistence of the warming over the past 60 years far exceeds what can be accounted for by natural variability alone. That does not mean, of course, that natural sources of variability have become insignificant. They can be expected to continue to contribute a degree of “bumpiness” in the year-to-year global average temperature trajectory, as well as exert influences on the average rate of warming that can last a decade or more (see Box 1.1)….
The frequency of multiday heat waves and extreme high temperatures at both daytime and nighttime hours is increasing over many of the global land areas. There are increasing areas of land throughout our planet experiencing an excess number of daily highs above given thresholds (for example, the 90th percentile), with an approximate doubling of the world’s land area since 1998 with 30 extreme heat days per year. At the same time, frequencies of cold waves and extremely low temperatures are decreasing over the United States and much of the earth. In the United States, the number of record daily high temperatures has been about double the number of record daily low temperatures in the 2000s, and much of the United States has experienced decreases of 5%–20% per decade in cold wave frequency. …
Observations continue to show declines in arctic sea ice extent and thickness, Northern Hemisphere snow cover, and the volume of mountain glaciers and continental ice sheets. , , , , , Evidence suggests in many cases that the net loss of mass from the global cryosphere is accelerating indicating significant climate feedbacks and societal consequences. …
Nearly two-thirds of the sea level rise measured since 2005 has resulted from increases in ocean mass, primarily from land-based ice melt; the remaining one-third of the rise is in response to changes in density from increasing ocean temperatures….”
Perhaps the most significant conclusions are:
“Beyond the next few decades, the magnitude of future climate change will be primarily a function of future carbon emissions and the response of the climate system to those emissions. … Even though natural variability will continue to occur, most of the difference between present and future climates will be determined by choices that society makes today and over the next few decades. The further out in time we look, the greater the influence of these human choices are on the magnitude of future warming….”