The Fourth US National Climate Assessment (NCA4) was released on Friday. It clearly states that climate change is real, it is happening now, and human activities are the main cause.
The first volume of the assessment – the Climate Science Special Report – says the average global temperature has increased 1.8°F (1°C) during the past 115 years (1901-2016). “This period is now the warmest in the history of modern civilization,” according to the report. It also says that “it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century.” Going even further, the report concludes, “there is no convincing alternative explanation supported by the extent of the observational evidence.”
With a focus on the US, the report says the average annual temperature in the contiguous forty-eight states has increased by 1.2°F (0.7°C) since 1986, relative to the previous century. It is projected to increase 2.5°F (1.4°C) by 2050.
In addition to warming, other aspects of climate change are highlighted in the massive report. One of these is sea level rise. Since 1900, the average global sea level has gone up 8 inches. Of that total, a 3-inch rise has occurred since 1993. This rate of rising, according to the report, “is greater than during any preceding century in at least 2,800 years.”
Moving forward, all projections show sea level continuing to rise. An increase of several inches is likely in the next fifteen years and 1 to 4 feet is estimated by 2100. A rise of as much as 8 feet by the end of the century, however, “cannot be ruled out”, the report warns. This is especially true if the ice sheets in Greenland and Antarctica prove to be more sensitive to rising temperatures than expected. In cities along the Gulf Coast and Eastern Seaboard, where daily tidal flooding is already a problem, sea level rise is forecast to be even higher than the global average.
The report also contains details on how extreme weather is changing across the US. For example, heavy rainfall events are ”increasing in intensity and duration” nationwide with the biggest increases happening in the northeast. In the west, the incidence of large forest fires has been rising since 1980 and is expected to increase even further in the coming years. Heat waves, unsurprisingly, have also become more frequent while cold waves have become less frequent.
Looking beyond the next few decades, the NCA says the magnitude of climate change depends on the amount of greenhouse gases that are added to the atmosphere. The level of carbon dioxide in the air today has already passed 400 parts per million, a number not seen in 3 million years. If emissions are not reined in, the average global temperature could increase by as much as 9°F (5°C) by the end of this century. According to the report, “the further and the faster the Earth system is pushed towards warming, the greater the risk of unanticipated changes and impacts.”
Mandated by Congress under the Global Change Research Act of 1990, this exhaustive climate report was produced by hundreds of experts from government agencies as well as academia and was peer-reviewed by the National Academy of Sciences. Published every four years, it is considered this country’s most authoritative statement on climate change.
Its findings, however, are in stark contrast to the words and actions of the Trump Administration. The President has called climate change a “hoax” and in June announced that he is withdrawing the US from the Paris Climate Agreement – an international accord that aims to reduce greenhouse gases and limit global warming to 2°C (3.6°F). The US and Syria are now the only two countries not part of the historic agreement.
Global annual average temperature (left) and surface temperature change for the period 1986–2016 relative to 1901–1960. Credit: NCA4