The Azimuth Project
Global warming (Rev #48)


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:

Mauna Loa CO2
Image source: Global Warming Art (click for data details)

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:

GISTEMP temperature data
Image source: NASA GISS Surface Temperature Analysis (click for data details)

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:

Image source: Global Warming Art (click for data details)

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:

Arctic sea ice levels
Image source: National Snow and Ice Data Center (NSIDC) (click for data details)

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:

CO2-temperature relation
Image source: BASC Climate Stabilization Targets (click for data details)

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:

Stern Review graphic
Image source: Stern Review on the economics of climate change (click for data details)

Other pages

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.

pop vs co2 levels


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:

  • David Archer and Ray Pierrehumbert, editors, The Warming Papers, Wiley-Blackwell, 2011.

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:

category: climate, carbon