Humans responsible for heating the Earth, US report says

The Earth is hotter, climate change is real, and humans are the most likely cause, according to a report released Friday by the US government. 

The federally mandated Climate Science Special Report is the first of two volumes prepared by the country's top scientists for the president, Congress and the public. The report was prepared by hundreds of scientists, who examined more than 1,500 scientific studies and reports to write it. This is the fourth National Climate Assessment, which must be published every four years, according to the Global Change Research Act of 1990. 

"The Climate Science Special Report lays out the most recent scientific evidence of climate change, once again confirming that climate change is real, it's happening now, and human activity is the primary cause," Rush Holt, CEO of the American Association for the Advancement of Science, said in a statement. 

The last three years have been the warmest on record, according to the report, which expects climate-related extremes to continue. The federal report also makes clear that human activity is responsible for climate change. It was peer reviewed by the National Academy of Sciences. The White House Office of Science and Technology signed off on it.

This assessment concludes, based on extensive evidence, 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. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.

The scientists' findings include:

• Sea level around the world has risen by 7 to 8 inches since 1900. Almost half of that occurred since 1993. "Human-caused climate change has made a substantial contribution to this rise since 1900, contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years."
• Rainfalls are becoming more extreme and more frequent in the US, especially in the Northeastern United States. 
• The US will likely experience more heatwaves and large forest fires. 
• The severity of future climate change will depend primarily on greenhouse gas emissions, especially carbon dioxide. 

The amount of carbon dioxide in the Earth's atmosphere "has 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," according to the report. "Continued growth in CO2 emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens to hundreds of millions of years."