The average global temperature is rising. Over the 20th century, the global surface temperature increased by about 0.6°C and the 20th century was probably the warmest century in the last 1,000 years. The rate of change for the period since 1976 is roughly three times that for the past 100 years as a whole.
The year 2004 is placed as the fourth warmest year, just behind 2003, and with 1998 as the warmest year since 1861 when recording started (WMO).
Other evidences of a changing climate are retreating glaciers, thinning of arctic sea-ice, changed rainfall patterns, rising sea levels and the increasing incidence of extreme weather in some parts of the world. Extreme weather such as hurricanes, typhoons, abundant rainfall and flooding, drought and abnormally cold or warm conditions has been reported frequently.
The observed changes in global climate during the 20th century are likely to be due to a combination of both natural and human causes (anthropogenic). Interactions between the ocean and the atmosphere, changes in the Earth's orbit, fluctuations in energy received from the sun and volcanic eruptions are natural causes. Burning of fossil fuels, which leads to an increase of atmospheric greenhouse gases such as carbon dioxide and methane, are human causes. Fossil fuels are coal, oil and gas for energy. Changes in land use also lead to further emissions.
The concept of radiative forcing is used to describe the influence of external factors. It describes the influence of a factor altering the Earth-atmosphere system energy balance expressed in Watts per square metre (W/m2). Radiative forcing arises from changes in the atmospheric composition, alteration of surface reflectance by land use, and variation in the output of the sun.
Increased concentrations of greenhouse gases tend to warm the surface and are thus a positive radiative forcing. The forcing agents are well mixed and long-lasting. Increased concentrations of some types of aerosols tend to cool the surface and are thus a negative radiative forcing. Anthropogenic aerosols are short-living with a strong regional signature since they vary considerably by region. The radiative forcing of aerosols is not fully understood regarding their indirect effect on clouds. Natural factors, such as changes in solar output or volcanic eruptions, can also cause both negative and positive radiative forcing. Forcing due to episodic volcanic events lasts only a few years.
The additional heat flux from the atmosphere back to the surface resulting from anthropogenic greenhouse gases has been estimated to about 2.5 W/m2 (IPCC, 2001). According to the Intergovernmental Panel on Climate Change (IPCC), most of the observed warming over the last 50 years can be identified as attributed to anthropogenic greenhouse gases.
Other evidences of a changing climate are retreating glaciers, thinning of arctic sea-ice, changed rainfall patterns, rising sea levels and the increasing incidence of extreme weather in some parts of the world. Extreme weather such as hurricanes, typhoons, abundant rainfall and flooding, drought and abnormally cold or warm conditions has been reported frequently.
Natural and anthropogenic causes
The climate of the Earth has changed many times historically in response to different natural causes of which we do not fully understand the processes. These relatively rapid changes indicate that the climate is quite sensitive to internal or external climate forcing and feedbacks.The observed changes in global climate during the 20th century are likely to be due to a combination of both natural and human causes (anthropogenic). Interactions between the ocean and the atmosphere, changes in the Earth's orbit, fluctuations in energy received from the sun and volcanic eruptions are natural causes. Burning of fossil fuels, which leads to an increase of atmospheric greenhouse gases such as carbon dioxide and methane, are human causes. Fossil fuels are coal, oil and gas for energy. Changes in land use also lead to further emissions.
Radiative forcing
The Earth is heated by solar irradiance, about 342 W/m2 as a global average. A part of this flow is directly reflected back to space and a part is absorbed in the atmosphere. The largest part is absorbed by the Earth's surface. At any given temperature, the surface emits thermal radiation. Most of this radiation is absorbed by (natural and anthropogenic) greenhouse gases in the atmosphere.The concept of radiative forcing is used to describe the influence of external factors. It describes the influence of a factor altering the Earth-atmosphere system energy balance expressed in Watts per square metre (W/m2). Radiative forcing arises from changes in the atmospheric composition, alteration of surface reflectance by land use, and variation in the output of the sun.
Increased concentrations of greenhouse gases tend to warm the surface and are thus a positive radiative forcing. The forcing agents are well mixed and long-lasting. Increased concentrations of some types of aerosols tend to cool the surface and are thus a negative radiative forcing. Anthropogenic aerosols are short-living with a strong regional signature since they vary considerably by region. The radiative forcing of aerosols is not fully understood regarding their indirect effect on clouds. Natural factors, such as changes in solar output or volcanic eruptions, can also cause both negative and positive radiative forcing. Forcing due to episodic volcanic events lasts only a few years.
The additional heat flux from the atmosphere back to the surface resulting from anthropogenic greenhouse gases has been estimated to about 2.5 W/m2 (IPCC, 2001). According to the Intergovernmental Panel on Climate Change (IPCC), most of the observed warming over the last 50 years can be identified as attributed to anthropogenic greenhouse gases.