Comparison Of Shortterm Climate Changes With The mediumterm paleoclimate RECORD

separating the effects of short-term human-induced climate changes from natural variations on longer-term timescales can be difficult. Present-day global warming is unusual for the climate record of the past 1,300 years, but it has counterparts induced by natural causes about 125,000 years ago and in the older geological record. The last time (125,000 years ago) climates warmed as significantly as the planet is now experiencing, loss of polar ice led to sea-level rise of 13-20 feet (4-6 m), suggesting that the world's coastlines are in grave danger of moving inland to higher ground. Ice core data show that temperatures in Greenland were 4-7°F (3-5°C) hotter than at present, a level that many models predict will be reached by the end of this century. The last 50 years appear to be the hottest in the past 1,300 years, but significant fluctuations have occurred.

The measured increases in anthropogenic greenhouse gases can more than account for the measured temperature rise of the surface of the Earth in the past 50-100 years. The less-than-expected warming is probably related to lowering of the temperature by aerosols from volcanic eruptions and dust from desert environments. These measurements strongly suggest that present-day global warming is being forced by the human-induced injection of greenhouse gases to the atmosphere, not to other long-term climate-forcing mechanisms that have controlled other global warming and cooling events in past geological times.

The measured surface warming is nearly global in scale, with Antarctica the exception; it is sheltered from parts of the global atmosphere/ocean system. Climate models are consistent with the global warming produced by anthropogenic causes. Many local variations exist, such as "warming holes," where local atmospheric effects are stronger than the global changes.

Global warming is also likely affecting wind patterns, the most extreme hot and cold nights, extratropical storm patterns, and an increase in heat waves. Effects are stronger in the Northern than in the southern Hemisphere.

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