The main evidence for global warming comes from thermometer measurements from land stations all over the world since 1860. The data, corrected for the urban heat island effect, shows that the average surface temperature has increased by 0.6±0.2 C during the 20th century. Most of the warming occurred during two periods: 1910 to 1945 and 1976 to 2000. (Source: IPCC)
Secondary evidence comes from observed variations on the snow cover and ice extent, global average sea level, precipitation, cloud cover, [El Nino]? and extreme weather events during the 20th century. For example, satellite data shows a 10% decrease of snow cover since the late 1960s. The Northern Hemisphere spring and summer sea-ice extent has decreased by about 10% to 15% since the 1950s and there has been a widespread retreat of mountain glaciers in non-polar regions during all the 20th century. (Source: IPCC).
Climate changes occur due to internal and external factors. Internal factors are factors associated with the complexity of the climate systems which are chaotic non-linear systems. External factors can be natural factors and anthropogenic factors. The main natural external factor is the solar radiation variability. The solar radiation can change because of the solar cycles and because the Sun is getting hotter. Anthropogenic factors are related to changes in the environment influenced by Man. The main anthropogenic factors are the emission of greenhouse gases, the depletion of stratospheric ozone, the land use and the emission of aerosols? such as sulphates?. (Source: IPCC)
Scientists agree that internal factors and natural external factors can cause significant climate changes. In the last millennium, two important periods occurred: a warm period known as the [Medieval Warming Period]? and a cold period known as the [Little Ice Age]?. These periods have a magnitude similar to the current warming and it's agreed that they were cause by internal factors and natural external factors only. The Little Ice Age is usually attributed to the reduction of solar activity. Some scientists have claimed in the past that the observed warming since 1860 is a natural climate recovery from the Little Ice Age. (Source: The Skeptical Environmentalist)
However, large amounts of anthropogenic greenhouse gases were emitted to the atmosphere since the beginning of the industrial revolution. Since 1750, the carbon dioxide concentration has increased by 31%, methane has increased 151%, [nitrous oxide]? has increased 17% and tropospheric? ozone has increased 36%. (Source: IPCC)
The majority of these gases are produced by the combustion of fossil fuels. It is thought that the reduction in tropical forested area has also played a role, as old forests store large amounts of carbon. However, growing forests in North America and Russia contribute to absorb carbon dioxide (they act as CO2 sinks) and since 1990, the amount of carbon absorbed is larger than the amount released by deforestation. Not all the CO2 emitted to the atmosphere accumulates there. Half of it is absorbed by sea and forests.
The real importance of each of the proposed causes can only be established through the exact quantification of every factor involved. Internal and external factors can be quantified by the analysis of climate simulations based on the best [climate models]?.
The influence of external factors can be compared using the concept of [radiative forcing]?. A positive radiative forcing warms the planet and negative radiative forcing cools the planet. Anthropogenic emissions of greenhouse gases, stratospheric ozone depletion, and solar radiation have positive radiative forcing and aerosols? and land use have negative radiative forcing. (Source: IPCC)
Climate simulations show that warming occurring from 1910 to 1945 can be explained by internal and natural forcing (variation in solar radiation) only, but warming occurring from 1976 to 2000 needs anthropogenic greenhouse gases emissions to be explained. The majority of the scientific community is now convinced that a significant proportion of observed global warming is caused by anthropogenic emissions of greenhouse gases. (Source: IPCC)
This conclusion depends on the accuracy of the models used and on the correct estimation of the external factors. The majority of scientists agrees that important climate features are incorrectly accounted by the [climate models]?, but these scientists don't think that better models would change the conclusion. (Source: IPCC)
Critics point out that there are flaws in the models and external factors not taken into consideration that could change the conclusion above. The critics say that the climate simulations are unable to model the cooling effects of the particles, fitting the water vapor feedback and handling clouds. Critics also point out that the Sun may have a share of responsibility for the observed global warming greater than now thought by the majority of the scientific community. Some indirect solar effects may be very important and are not accounted by the models. So, the share of global warming caused by anthropogenic greenhouse gases may be lower than thought. (Source: The Skeptical Environmentalist)
Due to potential effects on human health, economy and the environment, global warming is the cause of great concern. Some important environmental changes have been observed and were linked to global warming. The examples of secondary evidence cited above (lessened snow cover, rising sea levels, weather changes) are examples of consequences of global warming that may influence not only human activities but also the ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions while others may spread.
However, global warming can also have positive effects, since higher temperatures and higher CO2 concentrations improve the ecosystems' productivity. Satellite data shows that the productivity of the Northern Hemisphere increased since 1982. On the other hand, just the fact that the total amount of biomass produced increases is not necessarily all good, since biodiversity can still decrease even though a small number of species are flourishing.
Another cause of great concern is sea level rise. The sea levels are rising 0.01 to 0.02 meters per decade and some small countries in the Pacific Ocean are expressing concerns that if this rise in sea level doesn't stop, they soon will be entirely under water. Global warming causes the sea level to rise mainly because sea water expands, but some scientists are concerned that in the future, the polar ice caps and glaciers may melt. As a consequence, the sea level could rise several meters. At the moment, scientists are not expecting any major ice melting in the next 100 years. (Sources: IPCC for the data and The Mass Media for the general perception that climate change is important)
As the climate gets hotter, evaporation? will increase. This will cause heavier rainfall? and more erosion. Many people think that it could result in more extreme weather as global warming progresses.
Global warming can also have other, less obvious effects. The North Atlantic drift, for instance, is driven by temperature changes. It seems as if it is diminishing as the climate grows warmer, and this means that areas like Scandinavia and Britain that are warmed by the drift might face a colder climate in spite of the general global warming.
Since it is such an important issue, governments need predictions of future trends in global change so they can take political decisions to avoid undesired impacts. Global warming is being studied by the Intergovernmental Panel on Climate Change (IPCC). In its last report, IPCC made some predictions about future climate change. These predictions are the basis for current political and scientific discussion.
IPCC predictions are based on the same models used to establish the importance of the different factors in global warming. These models have the need for data of anthropogenic emissions of greenhouse gases and aerosols. This data is predicted from economical models based on 35 different scenarios. Scenarios go from pessimistic to optimistic and predictions of global warming depend on the kind of scenario considered. No scenario considers any kind of measures to avoid global warming.
In it's last report, IPCC stated that average surface temperature is projected to increase by 1.4 to 5.8 C over the period 1990 to 2100, and the sea level is projected to rise by 0.1 to 0.9 meters over the same period.
IPCC prediction are the best predictions available but are under strong scientific controversy. The IPCC recognizes that there is a need for better models and better scientific understanding of some climate phenomena, as well as the uncertainties involved. Critics point out that the available data is not sufficient to determine the real importance of greenhouse gases in climate change. Sensibility of climate to greenhouse gases may be overestimated because of some flaws in the models and because the importance of some external factors may be underestimated.
On the other hand, predictions are based on scenarios and the IPCC did not assign any probability to the 35 scenarios used. Critics point out that some of the scenarios that predict the largest impacts are not realistic because they contradict basic economical reasoning.
Even if there are some doubts about its importance, global warming is perceived by the general public and by some political leaders as a potential threat. Reductions of the emissions of greenhouse gases was proposed. Being a tragedy of the commons like problem, only an international agreement could reduce the emissions of greenhouse gases since voluntary reductions would be avoided by individual countries. The Kyoto protocol to the United Nations Framework Convention on Climate Change was signed by all industrialized countries who agreed to reduce emissions below the 1990 level. It was agreed that developing countries would be out of the agreement. The United States, responsible for 1/3 of the emissions of greenhouse gases, left the Kyoto protocol before its final ratification by Congress. President Bush's new administration (2001) took the decision of leaving the United States out. This decision resulted in internal and international controversy with major political and ideological ramifications.
To evaluate the effectiveness of the Kyoto protocol, it is necessary to compare global warming with and without the agreement. Several independent authors agree that the impact of the Kyoto protocol in global warming is very small (a reduction of 0.15 in 2 C warming by 2100). Even some defenders of the Kyoto protocol agree that the impact of it is small, but they view it as a first step, with more political than practical importance, for future reductions. At the moment, an analysis made by the IPCC is needed to clarify this issue.
The Kyoto Protocol can also be evaluated by comparing costs and gains. Several economical analyses were made that show that the Kyoto Protocol is more expensive than the global warming that it avoids. Defenders of the Kyoto Protocol argue however that while the initial greenhouse gas cuts may have little effect, they set the political precedent for bigger (and more effective) cuts in the future.
See also: Global warming potential
Scientific Discussion:
Political Discussion:
Because global warming is a controversial issue, every source of information has been accused, in one time or another, of having some kind of bias.
IPCC is accused by critics for exaggerating the dangers of global warming. Critics point out that IPCC needs to prove that global warming is happening and is important because they want more funds. However, even some critics agree that IPCC has the best information available.
Bjorn Lomborg (The Skeptical environmentalist) is a Danish professor, ex-Greenpeace, that seems to represent only himself. It can be pointed out that professor Bjorn Lomborg is looking for fame and fortune.
SEPP? maintains that it is scientific, rather than political, and that IPCC is politically biased. See www.sepp.org
Junkscience claims to uncover error and bias in science reporting (see Junk science). It has links to the [Cato Institute]?, an organization that promotes the [free market]?. See www.junkscience.com
Greenpeace and the World Wide Fund for Nature have been accused by critics of promoting fear to promote their political agenda.
![[Home]](wikipedia.png)