Globalwarming awareness2007

The extent of the scientific consensus on global warming—that “most of the observed warming over the last 50 years is likely to have been attributable to human activities”—has been investigated: In the journal Science in December 2004, Dr Naomi Oreskes published a study of the abstracts of the 928 refereed scientific articles in the ISI citation database identified with the keywords “global climate change” and published from 1993–2003. This study concluded that 75% of the 928 articles either explicitly or implicitly accepted the consensus view—the remainder of the articles covered methods or paleoclimate and did not take any stance on recent climate change. The study did not report how many of the 928 abstracts explicitly accepted the hypothesis of human-induced warming, but none of the 928 articles surveyed accepted any other hypothesis[1].

Contrasting with the consensus view, other hypotheses have been proposed to explain all or most of the observed increase in global temperatures. Some of these hypotheses (listed here without comment on their validity or lack thereof) include:

• The warming is within the range of natural variation.
• The warming is a consequence of coming out of a prior cool period, namely the Little Ice Age.
• The warming is primarily a result of variances in solar irradiance, possibly via modulation of cloud cover. It is similar in concept to the operating principles of the Wilson cloud chamber, but on a global scale where earth’s atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of cloud condensation nuclei.
• The observed warming actually reflects the Urban Heat Island, as most readings are done in heavily populated areas which are expanding with growing population.

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The climate system varies both through natural, “internal” processes as well as in response to variations in external “forcing” from both human and non-human causes, including solar activity, volcanic emissions, and greenhouse gases. Climatologists agree that the earth has warmed recently. The detailed causes of this change remain an active field of research, but the scientific consensus identifies greenhouse gases as the primary cause of the recent warming. Outside of the scientific community, however, this conclusion can be controversial.

Adding carbon dioxide (CO₂) or methane (CH₄) to Earth’s atmosphere, with no other changes, will make the planet’s surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C (54 °F) lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who “believe in” and “oppose” the theory that adding carbon dioxide or methane to the Earth’s atmosphere will, absent any mitigating actions or effects, result in warmer surface temperatures on Earth. Rather, the debate is about what the net effect of the addition of carbon dioxide and methane will be, when allowing for compounding or mitigating factors.

One example of an important feedback process is ice-albedo feedback. The increased CO₂ in the atmosphere warms the Earth’s surface and leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues.

Due to the thermal inertia of the earth’s oceans and slow responses of other indirect effects, the Earth’s current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilized at present day levels, a further warming of perhaps 0.5 °C to 1.0 °C (0.9–1.8 °F) would still occur.

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Relative to the period 1860–1900, global temperatures on both land and sea have increased by 0.75 °C (1.4 °F), according to the instrumental temperature record. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C/decade against 0.13 °C/decade (Smith, 2005). Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C per decade since 1979, according to satellite temperature measurements. Over the one or two thousand years before 1850, world temperature is believed to have been relatively stable, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Based on estimates by NASA’s Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree. Estimates prepared by the World Meteorological Organization and the UK Climatic Research Unit concluded that 2005 was still only the second warmest year, behind 1998.

Depending on the time frame, a number of temperature records are available based on different data sets. The longest perspective is available from various proxy records for recent millennia; see temperature record of the past 1000 years for a discussion of these records and their differences. An approximately global instrumental record of temperature near the earth’s surface begins in about 1860. Global observations of the atmosphere well above the earth’s surface using data from radiosondes began shortly after World War II. Satellite temperature measurements of the tropospheric temperature date from 1979. The attribution of recent climate change is clearest for the most recent period of the last 50 years, for which the most detailed data are available.

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