While Earth's climate is constantly changing in various ways, the planet tends to experience long-term trends toward either warming or cooling. The potential or actual contribution of postindustrial human activity to climate change, the consequences of that contribution, and the proper response to those consequences remain matters of crucial importance and signficant controversy.

Climate is characterized by mean air temperature, humidity, winds, precipitation, and frequency of extreme weather events over a lengthy period of time, at least thirty years. Global warming is an example of climate change, and so are increases in the magnitude or frequency of floods and droughts experienced in many parts of the world during the past several decades. Climate change includes both natural variability and anthropogenic changes.

Although climate changes on longer than millennial timescales are natural, the global warming of the past 150 years or so is likely anthropogenic, according to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) of the United Nations. The United Nations is concerned primarily with anthropogenic climate change, both because it poses a threat to global security and because it can be altered by altering human and governmental behavior. For this reason, the United Nations Framework Convention on Climate Change (UNFCCC) defines climate change as a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.

Earth's atmosphere is chaotic, and weather can change dramatically in a matter of days or even hours. Temperature in some places may rise or fall by 20œ Celsius or more in one day. On the other hand, climate, as the average of years of weather conditions, changes on a much smaller scale. For example, the global mean surface air temperature increased by only 0.6œ Celsius during the twentieth century. By the same token, such a seemingly small increase can have extremely significant effects.

The climatic increase in mean surface air temperature is computed from tens of thousands of weather station records spanning decades. The difficulty of ensuring data continuity in time, uniformity in space, and constancy in observational methods poses serious challenges to climatologists. To discern slight trends amid diverging data, scientists use advanced mathematical tools to synchronize all observations, adjust discontinuities, and filter out local influences such as heat island effects.

Modern climate change has generally been observed with in situ thermometers and, later, with remote sensing devices. Paleoclimate change (change before about 1850) is inferred from proxy climate data. Tree rings can provide evidence of temperature and precipitation history for two to three thousand years, while tiny air bubbles trapped in the Antarctic ice deposits provide data on ice ages hundreds of thousands of years in the past. Pollen and  zooplankton cells in river and sea sediments also contain useful proxy climate data.

Detecting climate change depends on individual variables. Temperature change is the most reliable such variable, because its internal variability is small and it is more widely observed than other variables. Long-term precipitation changes are more difficult to discern, because rain- and snowfall vary so greatly from one year to the next. The intensity and frequency of extreme weather events such as hundred-year floods are even more difficult to detect, because these events are rare, so a significant data set must cover many years.

Instrument records from land stations and ships indicate that the global annual mean surface air temperature rose during the twentieth century. The warming occurred more quickly in high latitudes than it did in the tropics. It was also faster over land than it was over the ocean and faster in the Northern Hemisphere than in the Southern Hemisphere. Winters warmed more than did summer, and nights warmed more than did days. Contemporary daily temperature ranges have narrowed, precisely because nights have warmed more than have days.

Extensive heat waves and intense floods have become more frequent in recent decades. Globally, the average number of tropical storms (about ninety per year) changed little during the twentieth century, although historical data are poor for some regions. In the North Atlantic, where the best records are available, there has been a clear increase in the number and intensity of tropical storms and major hurricanes. From 1997 to 2006, there were about fourteen tropical storms per year, including about eight hurricanes in the North Atlantic, compared to about ten storms and five hurricanes between 1850 and 1990.

On timescales of thousands of years or greater, the Earth's climate has been both warmer and much colder than it is today, although temperatures around the turn of the twenty-first century were the warmest in the past two thousand years. Based on ice-core proxy data, four major global glaciations occurred in past 450,000 years, about one every 100,000 years, correlating well with the cyclical variations in Earth's orbit known as the Milankovic cycles. Various ice ages occurred, with the most recent one ending about 11,500 years ago. Before that, much of North America was covered in permanent ice. Over the course of Earth's history, its temperature has swung more than 10œ Celsius between cold and warm modes.

Future climate changes are predicted by climate models based on assumed greenhouse gas (GHG) emission scenarios. The scenarios range from high fossil fuel consumption, resulting in atmospheric carbon dioxide (CO2) concentration of 800 parts per million, to low consumption, with CO2 concentration reaching 550 parts per million. The reliability of these predictions depends on future global environmental, energy, and climate policy, as well as the accuracy of the models.

Most models project that climate change will accelerate during the twenty-first century and that the global average temperature will increase by between 1.8œ Celsius and 4.0œ Celsius by 2100. As in the past, warming will be more pronounced in the polar Northern Hemisphere during winter. Precipitation amounts are likely to increase in high latitudes and to decrease in most subtropical lands. Heat waves and heavy precipitation events will very likely increase in frequency. With warmer oceans, future tropical storms will become more intense, with greater peak wind speeds and heavier precipitation.

Climate change may be attributed to natural processes or to human activity. Natural factors include the Earth's internal processes, such as volcanic eruptions, as well as external parameters, such as solar luminosity and Earth's orbital pattern around the Sun. Anthropogenic activity includes GHG and aerosol emission and, to a lesser degree, changes in land use. Separating natural and anthropogenic causes of climate change is challenging, if it is possible at all. Since no controlled laboratory setting exists in which to conduct climate change experiments, climate scientists have developed computer models based on the laws governing climate systems. By altering model settings, one can simulate natural and anthropogenic effects on climate, separately or in combination, thereby tracing the causes of climate change. In general, on scales of a decade to a century, climate change is attributable to atmosphere-ocean interaction and to human activity. On scales of millennia to hundreds of thousands of years, the variations in Earth's orbit directly controls the planet's climate. This orbit is described by the Milankovic cycles, which repeat every 20,000 to 100,000 years. Beyond the million-year timescale, tectonic drift is likely the main driver of climate change.

References:

1)         Easterling, David R., et al. "Maximum and Minimum Temperature Trends for the Globe." Science 277 (July 18, 1997): 364-367.

2)         Intergovernmental Panel on Climate Change. Climate Change, 2007--The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007.

3)         Mann, Michael E., Raymond S. Bradley, and Malcolm K. Hughes. "Global-Scale Temperature Patterns and Climate Forcing over the Past Six Centuries." Nature 392 (April 23, 1998): 779-787.

4)         Nakicenovic, N., et al., eds. Special Report on Emission Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2000.

5)         National Research Council. Abrupt Climate Change: Inevitable Surprises. Washington, D.C.: National Academies Press, 2002.

6)         United Nations Framework Convention on Climate Change, Article 1: Definitions. Geneva, Switzerland: United Nations, 1992.

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