Global warming refers to an ongoing increase in the global average annual temperature. This page is an overview for global warming.
Current-day global warming is sometimes called anthropogenic (due to human causes) global warming because it is caused by the increase in carbon emissions and other greenhouse gases from burning fossil fuels; deforestation; agriculture; and other human activity. In a process known as the “greenhouse effect”, greenhouse gases in the atmosphere—which are largely transparent to the higher-frequency radiation from the Sun—re-radiate back towards the Earth a portion of the lower-frequency infrared radiation emitted by the warm surface of the planet. This makes the Earth’s temperature much higher than it would be without greenhouse gases. The planet’s temperature is regulated by the concentration of greenhouse gases in the atmosphere, so an increase in greenhouse gas concentration will cause an increase in global average annual temperature. Therefore one of the major efforts to combat global warming is the effort to reduce net greenhouse gas emissions.
Climatology scientists reached a consensus in the 1970s and 1980s that global warming represents a serious threat to humans and the ecosystems of the planet. Subsequent events have validated that consensus and confirmed this threat.
In the later half of the 19th century scientists noticed that the Earth’s atmosphere kept it warm. In 1859, John Tyndall discovered that two major atmospheric gases H2O and CO2 could trap heat like the glass in a greenhouse. In 1896, Svante Arrhenius in Stockholm calculated that the burning of coal could increase the temperature of the Earth by 5-6°C if CO2 levels were doubled. At the rate of burning in 1896, this would have taken thousands of years. But rates quickly increased over the next few decades.
In the 1950s and 1960s, scientists started to gain more accurate measures of the increase in CO2 concentrations. CO2 is being added to the atmosphere at an exponential rate that doubles about every 35 years. The following graph shows the longest running measurements taken at the Mauna Loa observatory:
So it was clear by the 1960s and 1970s that CO2 levels were rising, but there was still considerable debate over whether or not rising CO2 levels represented a problem. This debate was prolonged because of the cooling effect of smog (aerosol pollution) which counteracted some of the warming effect of CO2. As the anti-pollution efforts of the 1960s and 1970s began to reduce smog levels, the cooling from aerosol pollution dropped off. So temperatures started rising again. This can be seen in the following graph showing rising temperatures. These leveled off just after World War II—when Europe’s economies were still rebuilding and high smog levels masked the green house effect—and then started rising again in the mid-1970s as air became clearer:
By the end of the 1970s, a consensus formed that rising CO2 levels would cause global warming. So the debate shifted from whether or not global warming would occur to how much warming there might be, and to what the effects of that warming might be.
Ice core samples taken during the 1980s showed a very strong correlation between the level of CO2 and glacial cycles, as the following graph of CO2 concentrations and glacial cycles over the last 400,000 years shows:
Note also that the recent CO2 levels greatly exceed the levels corresponding with the warming which caused the end of historical ice-age periods.
With the ice-core data, rising global temperatures, and improved science modeling, by the late 1980s, there was a consensus among climate scientists that greenhouse gas-induced global warming was a real threat to the Earth for the 21st century. The consensus also held that human burning of carbon-based fossil fuels was the major contributor to this threat.
Recent confirmation of this warming includes the decline in Arctic sea ice:
So how warm might it get? The following graph shows estimates of the ranges of potential increases in temperature for various stable concentrations of CO2:
How much the planet warms is largely a function of our response to the global warming threat and the ultimate CO2 levels we reach. Extrapolations from current trends indicate warming on the upper end of the spectrum of possibilities.
Are there there any specific target levels predicted by climate studies that we can cite?
What will the warming mean for the Earth? Probable effects of various CO2e concentrations Carbon dioxide equivalent are shown here:
Some other relevant Azimuth Library pages for global warming:
From Carbon in the Geobiosphere:
The figure shows the growth of world population in 200 years, from 1800 to 2000, and the growth of CO2 emissions from fossil fuel burning and land-use practices. In this period, the global population increased 6-fold, but the industrial and land emissions increased 20-fold. The logarithmic scale of the ﬁgure shows that the population growth rate became faster in 1900 and again in 1950, following the end of World War II. However, the CO2 emissions were growing faster than the population even in the 19th century and their growth accelerated further in the 20th.
A 2004 paper by geologist and historian of science Naomi Oreskes summarized a study of the scientific literature on climate change. She concluded that there is a scientific consensus on the reality of anthropogenic climate change. She analyzed 928 abstracts of papers from refereed scientific journals between 1993 and 2003, listed with the keywords “global climate change”. Oreskes divided the abstracts into six categories: explicit endorsement of the consensus position, evaluation of impacts, mitigation proposals, methods, paleoclimate analysis, and rejection of the consensus position. 75% of the abstracts were placed in the first three categories, thus either explicitly or implicitly accepting the consensus view; 25% dealt with methods or paleoclimate, thus taking no position on current anthropogenic climate change; none of the abstracts disagreed with the consensus position, which Oreskes found to be “remarkable”. According to the report, “authors evaluating impacts, developing methods, or studying paleoclimatic change might believe that current climate change is natural. However, none of these papers argued that point.”
In 2007, Harris Interactive surveyed 489 randomly selected members of either the American Meteorological Society or the American Geophysical Union for the Statistical Assessment Service (STATS) at George Mason University. The survey found 97% agreed that global temperatures have increased during the past 100 years; 84% say they personally believe human-induced warming is occurring, and 74% agree that “currently available scientific evidence” substantiates its occurrence. Only 5% believe that that human activity does not contribute to greenhouse warming; and 84% believe global climate change poses a moderate to very great danger.
A poll performed by Peter Doran and Maggie Kendall Zimmerman in Earth and Environmental Sciences at the University of Illinois at Chicago received replies from 3,146 of the 10,257 polled Earth scientists. Results were analyzed globally and by specialization. 76 out of 79 climatologists who “listed climate science as their area of expertise and who also have published more than 50% of their recent peer-reviewed papers on the subject of climate change” believe that mean global temperatures have risen compared to pre-1800s levels, and 75 out of 77 believe that human activity is a significant factor in changing mean global temperatures. Among all respondents, 90% agreed that temperatures have risen compared to pre-1800 levels, and 82% agreed that humans significantly influence the global temperature. Economic geologists and meteorologists were among the biggest doubters, with only 47% and 64%, respectively, believing in significant human involvement. They write:
It seems that the debate on the authenticity of global warming and the role played by human activity is largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes.
A 2010 paper by Anderegg, Prall, Harold, and Schneider in the Proceedings of the National Academy of Sciences of the United States reviewed publication and citation data for 1,372 climate researchers and drew the following two conclusions:
97–98% of the climate researchers most actively publishing in the field support the tenets of ACC (Anthropogenic Climate Change) outlined by the Intergovernmental Panel on Climate Change, and
the relative climate expertise and scientific prominence of the researchers unconvinced of ACC are substantially below that of the convinced researchers.
In an October 2011 paper published in the International Journal of Public Opinion Research, Farnsworth and Lichter from George Mason University analyzed the results of a survey of 489 scientists working in academia, government, and industry. The scientists polled were members of the American Geophysical Union or the American Meteorological Society and listed in the 23rd edition of American Men and Women of Science, a biographical reference work on leading American scientists. Of those surveyed, 97% agreed that that global temperatures have risen over the past century. Moreover, 84% agreed that “human-induced greenhouse warming” is now occurring. Only 5% disagreed with the idea that human activity is a significant cause of global warming.
Global warming in the news - notable acknowledgements of global warming by newsmakers.
The greenhouse effect caused by CO2 has been a topic of scientific discussions for 200 years now, beginning with Fourier in 1827. The following volume collects some relevant papers that have been published in the course of time:
The physicist and science historian Spencer Weart has compiled an online history of global warming, which has also been condensed in book form:
It can be a bit difficult for beginners to gather and analyze climate data. For some useful tips, try:
The section on scientific opinions is based on this article:
Data on the global warming potential of greenhouse gases can be found at webpages