GLOBAL WARMING

Introduction

The phrase global warming refers to the documented historical warming of the Earth's surface based upon worldwide temperature records that have been maintained by humans since the 1880s. The term global warming is often used synonymously with the term climate change, but the two terms have distinct meanings. Global warming is the combined result of anthropogenic (human-caused) emissions of greenhouse gases and changes in solar irradiance, while climate change refers to any change in the state of the climate that can be identified by changes in the average and/or the variability of its properties (e.g., temperature, precipitation), and that persists for an extended period, typically decades or longer.





Global Mean Temperature over Land and Ocean (Jan-Dec).
According to the World Meteorological Organization (WMO), the decade of the 2000s (2000–2009) is the warmest on record. The global mean surface temperature for 2009 is currently estimated at 0.44°C/0.79°F above the 1961-1990 annual average of 14.00°C/57.20°F. WMO states that in 2009 above-normal temperatures were recorded in most parts of the continents. Only North America (United States and Canada) experienced conditions that were cooler than average.

The United States National Climatic Data Center (NCDC), found that in 2009:

Global land and ocean annual surface temperatures through December tied with 2006 as the fifth warmest on record as +0.56°C (+1.01°F) above the twentieth century average.
The 2000-2009 decade is the warmest on record, with an average global surface temperature of 0.54°C (0.96°F) above the 20th century average.
Ocean surface temperatures (through December) tied with 2002 and 2004 as the fourth warmest on record, at 0.48°C (0.86°F) above the 20th century average, and
and surface temperatures through December tied with 2003 as the seventh warmest on record, at 0.77°C (1.39°F) above the 20th century average.
The NCDC's Preliminary State of the Climate Global Analysis Annual Report of 2009  states that:

"The years 2001 through 2008 each rank among the ten warmest years of the 130-year (1880-2009) record and 2009 was no exception," and that
"during the past century, global surface temperatures have increased at a rate near 0.06°C/decade (0.11°F/decade), but this trend has increased to a rate of approximately 0.16°C/decade (0.29°F/decade) during the past 30 years".

CAUSES OF GLOBAL WARMING

In the IPCC Fourth Assessment Report scientists conclude that "warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level" and, furthermore, they conclude with "very high confidence (at least a 9 out of 10 chance of being correct) that the globally averaged net effect of human activities since 1750 has been one of warming" of the Earth's climate system.






Water in a boiling pot receives heat from an element or flame and loses heat via steam and radiative cooling.
As with every environmental variable, there are multiple factors that contribute to the "warmth" of the Earth. Humans measure warmth as temperature which is a measure of the amount of heat contained in a physical object. One can envision this concept by thinking of a pot on a stove. As heat is applied to the pot from a flame or heating element, the temperature of the pot will increase. But heat will also begin escaping the pot in the form of steam and also through radiative and convective cooling from the top and the sides of the pot. Eventually the rates of both heat loss (cooling) and heat gain (warming) may stabilize and the heat then contained within the pot at an instantaneous point of time would be reflected in an equilibrium temperature. This equilibrium temperature could be measured directly but it also could be calculated by determining all of the flux rates of heat entering (heating) and leaving (cooling) the pot.

One way that climate scientists look at the warmth of the Earth's climate system is to calculate the annual average temperature of the surface of the Earth using temperature measurements systematically collected throughout the year from thousands of land- and ocean-based weather and observation stations. The observed trends in the Earth's annual average temperature is one of the factors leading to the scientific conclusion that the Earth is now in a period of global warming.

In order to attempt to answer why the Earth is currently warming, scientists have conducted accountings of each of the fluxes of heat into (warming) and out of (cooling) the Earth's climate system. Since the measured data show that annual average temperatures of the Earth have been increasing in recent decades, the year-to-year annual flux of heat into the climate system must be greater than the annual flux of heat out of the system. By accounting for each of the fluxes of heat into and out of the system, scientists are able to assess which fluxes and processes are contributing to net annual warming of the Earth's surface. By conducting such accountings, scientists are able to quantify the influence that each natural and human factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and can calculate an index of the importance of each of the factor as a potential climate change mechanism. Each of the factors are called climate drivers and the relative impact or index of each factor's importance to climate change is called its radiative forcing.