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A scientific report on climate change obtained by The New York Times as part of a regular federal climate assessment, shows that warming is already having a large effect on the United States. By Henry Fountain and Nadja Popovich

1. IT'S HOT OUT THERE. It is getting warmer everywhere, but in the contiguous United States, the West is warming the fastest. While temperatures in the country (excluding Alaska and Hawaii) have increased an average of 1.2 degrees Fahrenheit since 1900, the Southwest and the Northwest, as well as the Northern Great Plains, have seen a temperature increase of 1.5 degrees or more. A degree and a half may not seem like much, but even slight changes in temperature can have widespread effects.

How average annual temperatures have changed since the first half of the 20th century.

The report said that heat waves and droughts had reached record intensities in some parts of the country. But the Dust Bowl of the 1930s remains the benchmark for heat and drought in American history, by virtue of the area involved and how long it lasted.

2. WETTER HURRICANES IN THE EAST. While it is not certain that the frequency of intense hurricanes will increase, hurricanes that do occur will bring more rainfall than ever and could potentially be more destructive.

3. BAD NEWS FOR CALIFORNIA. Warming will probably bring further reductions in winter and spring snowpack, which the state depends on for much of its water supply. If greenhouse gas emissions remain high and few steps are undertaken to better manage water resources, chronic long-lasting shortages — or hydrological drought — are possible by 2100.

4. WHEN IT RAINS IT POURS MORE. Americans are already experiencing more extreme precipitation. The amount of precipitation that falls in the heaviest storms is higher across the country, when storms from the last three decades are compared to storms from 1900 to 1960. The change has been highest in the Northeast, where 27 percent more rain falls in the worst storms.

5. FLOODING RELATED TO SEA-LEVEL RISE is a problem already. Tidal flooding that is already occurring in places like Miami and Norfolk, Va., will get worse throughout the century and affect cities on both coasts. By the end of the century, for example, parts of Charleston, S.C. may flood at high tide nearly every day. Low-lying parts of San Francisco are also extremely vulnerable, too, and are expected to have frequent flooding.

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6. SOME AREAS WILL SUFFER MORE from rising seas. Sea-level rise is expected to be worse in the Northeast and along the Gulf of Mexico, in part because the land in those regions is naturally subsiding. It is expected to be less of a problem in the Pacific Northwest and Alaska. Ocean circulation patterns, salinity and other factors can also influence how large the sea-level increase will be.

7. El Niño ISN'T FOREVER. Recurring natural shifts in atmospheric patterns can affect temperature and precipitation from months to years. The recent El Niño, for example, contributed to making 2015 and 2016 the warmest years on record. But such natural variability, the report said, has little influence on global or regional climate trends over periods of a decade or more.

8. HUMANS ARE TO BLAME. The federal report left no doubt as to responsibility for a warming climate: “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid 20th century.” There are no convincing alternative explanations supported by the observational evidence, the report said.

9. BE READY FOR SURPRISES in the future. The report said that through greenhouse gas emissions and widespread deforestation, humans were conducting an “unprecedented experiment” with the climate system. Although climate modeling has gotten more sophisticated, no model can capture all the elements of the earth’s complex climate.

The report warned there was a “significant possibility” of climate surprises in the future, either compound events, where two or more extreme events occur simultaneously, or tipping point events where some threshold in the climate system is crossed. The more the climate is changed — the more emissions continue or increase — the greater the risk of such surprises, the report said.

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