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). (Source: NCDC/NESDIS/NOAA)

According to the World Meteorological Organization (WMO), the decade of 1998-2007 is the warmest on record. The global mean surface temperature for 2007 is currently estimated at 0.41°C/0.74°F above the 1961-1990 annual average of 14.00°C/57.20°F. WMO states that among other remarkable global climatic events recorded in 2007, a record-low Arctic sea ice extent was observed which led to first recorded opening of the Canadian Northwest Passage.

The United States National Climatic Data Center (NCDC), found that in 2006 "Globally averaged land temperatures were +0.78°C (+1.40°F) and ocean temperatures +0.45°C (+0.81°F) above average, ranking 4th and 5th warmest, respectively. The land and ocean surface temperatures for the Northern and Southern Hemisphere ranked 2nd and 6th warmest, respectively," since global temperature record monitoring began in 1880. The NCDC report states 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 approximately 0.18°C/decade (0.32°F/decade) during the past 25 to 30 years. There have been two sustained periods of warming, one beginning around 1910 and ending around 1945, and the most recent beginning about 1976."

The NCDC's Preliminary Annual Report on the Climate of 2007 (released December 13, 2007) states that:

• "the global annual temperature for combined land and ocean surfaces for 2007 is expected to be near 58.0°F and would be the fifth warmest since records began in 1880," and that
  
• "the year 2007 is on pace to become one of the 10 warmest years for the contiguous U.S., since national records began in 1895."

Global in Situ Temperature Anomalies and Trends, Surface and Mid Troposphere (Jan-Dec).

The NCDC 2006 report also described temperature trends aloft in the atmosphere measured over the past 50 to 60 years using balloon-borne instruments (radiosondes) and for the past 28 years using satellites. The report states that temperature data collected from approximately 5,000 to 30,000 feet above the surface indicate that 1958-2006 global temperature trends in the middle troposphere are similar to trends in surface temperature; 0.12°C/decade for surface and 0.15°C/decade for mid-troposphere.

On 2 February 2007, the Intergovernmental Panel on Climate Change (IPCC) released the Summary For Policymakers (SPM), an executive summary of the first volume of its 4th Assessment Report entitled, "The Physical Science Basis of Climate Change." The IPCC Report documents that not only do the records show a warming trend during the past half century in land-based temperature data but also in global ocean temperature measurements. The increases in ocean temperatures indicate global warming trends are not an artifact of urbanization or the so-called "heat-island" effect.

U.S. Trends

The NCDC Report also documents not only a long-term warming trend for the globe as a whole but also a warming trend for the contiguous United States. The Report documents that the 2006 average annual temperature for the contiguous U.S. was the warmest on record and nearly identical to the record set in 1998.

National (Contiguous U.S.) Temperature (1895-2006). (Source: NCDC/NESDIS/NOAA)

Mean temperature values for the contiguous U.S. in the NCDC data set were calculated using a network of more than 1,200 U.S. Historical Climatology Network stations. These data, primarily from rural stations, have been adjusted to remove artificial effects resulting from factors such as urbanization and station and instrument changes which occurred during the period of record.

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.

Relative importance of climate drivers to current global warming as determined by the 4th Assessment of the IPCC. (Source: IPCC)

In completing such an assessment, the IPCC has concluded with very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming. The scientists found that the combined radiative forcing due to increases in carbon dioxide, methane, and nitrous oxide is the largest climate driver and its rate of increase during the industrial era is very likely to have been unprecedented in more than 10,000 years. Furthermore, the carbon dioxide radiative forcing increased by 20% from 1995 to 2005, the largest change for any decade in at least the last 200 years.

The IPCC also found that anthropogenic contributions to aerosols in the atmosphere produce cooling effects, referred to as global dimming. However the cooling (global dimming) effects due to human-caused aerosols are equivalent to about half of the warming effects due to the combined radiative forcing of human-produced greenhouse gases, causing a net warming.

Significant anthropogenic contributions to radiative forcing were also found to have come from several other sources, including tropospheric ozone changes due to emissions of ozone-forming chemicals, direct radiative forcing due to changes in halocarbons, and changes in surface albedo, due to land-cover changes and deposition of black carbon aerosols on snow. However the impacts of each of these factors was relatively small compared to the impacts of anthropogenic greenhouse gases (each showing relative impacts of 15% or less relative to the greenhouse gas forcings).

Finally, an increase in solar irradiance since 1750 was estimated to have caused a forcing that contributed to the recent warming of the Earth. However, the impact of the increase in the amount of sunlight striking the Earth each year during this ~250 year time span was estimated to be only about 1/20^th of the warming impacts of anthropogenic greenhouse gas emissions.

CLIMATE CHANGE

Climate change refers to a 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. Climate in a narrow sense is usually defined as the average weather, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization. Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use.

 

Figure 1. Observed changes in (a) global average surface temperature; (b) global average sea level from tide gauge (blue) and satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All differences are relative to corresponding averages for the period 1961-1990. Smoothed curves represent decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c). (Source: IPCC)

The temperature of the Earth has risen by about of 0.74 °C over the last century. While that may seem like a small increase, it has had profound effects on the planet's physical and biological systems, which, in turn, have impacted society. A large majority of the climate science community has very high confidence that the net effect of human activities since 1750 has been one of warming. They also conclude that most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas (GHG) concentrations.

Global GHG emissions will continue to grow over the next few decades due to increases in the human activities that generate GHG, notably the combustion of fossil fuels and certain land use practices. A pronounced and swift change in climate change mitigation policies and related sustainable development practices could mitigate that trend. Continued GHG emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century. Higher temperatures would cause further widespread change, including: a decrease in snow cover and sea ice; an increase in frequency of hot extremes, heat waves and heavy precipitation; an increase in tropical cyclone intensity; precipitation increases in high latitudes and likely decreases in most subtropical land regions, among many other impacts; sea level rise, and accelerated species extinction, among many other impacts.

These phenomena would have far-reaching impacts on society. Altered frequencies and intensities of extreme weather, together with sea level rise, are expected to have mostly adverse effects on natural and human systems. Projected impacts include increased pest outbreaks in agriculture, increasing water scarcity and diminished water quality, increased risk of heat-related mortality, relocation of coastal populations and infrastructure, and declining air quality in cities. There are sharp differences across regions and those in the weakest economic position are often the most vulnerable to climate change. Possible positive impacts of climate change include increased yields in colder environments and reduced energy demand for heating.

Some planned adaptation (of human activities) is occurring now; more extensive adaptation is required to reduce vulnerability to climate change. Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt. Making development more sustainable by integrating climate change adaptation and mitigation measures into sustainable development strategy, can make a major contribution towards addressing climate change problems. Although the problems are complex, we know enough today to take the first effective steps on adaptation and mitigation.

Further Reading

• 2006 Annual Report of the United States National Climatic Data Center (NCDC)
• Climate 2007: IPCC Fourth Assessment Report
• Preliminary 2007 Annual Report of the United States National Climatic Data Center (NCDC)
• World Meteorological Organization. December 13, 2007 Press Release.