Throughout the last few decades of the 20th Century, considerable debate ensued about whether increased levels of atmospheric CO2 due to human activity were responsible for warming the planet. However, by the end of the century, a large array of extremely diverse observations — from tree ring studies to general circulation models of the planet climate system to studies of glacier retreat — has produced a strong scientific case for anthropogenic global warming. Virtually all of the major scientific organizations, including the Intergovernmental Panel on Climate Change (IPCC), National Academy of Science, the American Geophysical Union, have issued reports and statements declaring that it is now unequivocal that the planet is heating up due, at least in part, to CO2 produced by human energy consumption.
That CO2 acts as a greenhouse gas is not even remotely contentious: the physics of this process has been known for well over 100 years. Note that the greenhouse effect is not inherently bad; in fact, without any greenhouse effect the average surface temperature of the Earth would be just 5°C (the so-called black-body temperature) or even colder if reflective cooling is included! In the distant geological past many tens of millions of years ago, CO2 concentration in the atmosphere was probably 2-5 times higher what it is today (Royer, 2006). However, in that "hothouse world", global sea level was also 100 ± 50 meters higher, and surface temperatures were considerably warmer, than today! Humans, and many of the species with whom we share the planet, evolved in an icehouse world with relatively cold temperatures, low sea level and low CO2. We, and our infrastructure and agriculture, are poorly adapted to hothouse conditions.
One of the most spectacular Earth Science success stories of the last two decades are the ice core records from Greenland and Antarctica. Annual layers of ice record a wealth of data about past climate, including capturing minute bubbles of trapped atmosphere, from which we can make direct measurement of past CO2 concentration. The records from Vostok and Dome Concordia ice cores demonstrate unequivocally, that at no time during the last 650,000 years did atmospheric CO2 concentration exceed about 300 ppm (Petit et al., 1999; Siegenthaler et al., 2005), even during interglacial periods when climate was as warm as today. During the last fifty years, careful weekly measurements of atmospheric CO2 show a systematic increase from about 315 ppm in 1958 to 380 ppm in 2005. The details of this curve, known as the Keeling curve after the scientist who initiated these measurements, show annual peaks and troughs due to the northern hemisphere growing season; the systematically changing distance between annual peaks shows that the growing season has increased by 10-12 days over the course of this record. Note that, due to the large heat capacity of the climate system, especially the oceans, there is a lag time of decades or longer between elevating the concentration of the green house gases, and the warming response of the climate system.
Global temperature has been increasing relatively steadily since the early 1800’s and the end of what was known in Europe as the Little Ice Age. Of the 1°C increase in average global temperature over that time interval, more than 50% has occurred since 1980. Over the last two decades, the increase in temperature has tracked closely the increase in atmospheric CO2. However, this is not true for the time period from the 1940’s to the end of the 1970’s when temperatures were relatively static. This time period figured prominently in Michael Crichton’s novel "State of Fear", his thinly veiled critique of global warming.
To understand this apparent lack of correlation, one needs to appreciate that there are, not one but, three main inputs that affect the surface temperature of a planet: (1) the so-called black body temperature due to incoming solar radiation, (2) the reflectivity of the planet, or its “albedo,” and (3) warming due to greenhouse gases. The first varies with minor changes in the Earth’s orbit and rotation axis and is, of course, completely beyond human control. Albedo is a function of the amount of snow and ice as well as the amount of particulates in the atmosphere due to anthropogenic aerosols as well as dust and volcanic eruptions. Albedo is particularly sensitive to feedbacks: for example, melting of Arctic sea ice reduces albedo and exposes more heat-absorbing sea surface, which raises air temperature which induces increased melting of permafrost on land which releases more green house gases into the atmosphere which causes more warming, etc. Finally, the main source of short term change in greenhouse gases is human activity, either directly through the burning of fossil fuels or indirectly through melting of permafrost, which contains substantial amounts of CO2 and methane. Modeling of historical climate data demonstrates that diminished solar insolation during the 1940’s to 1970’s was probably the prime cause for stagnant temperatures; the temperature increase since 1980 can only be modeled with significant increase in anthropogenic greenhouse gases.
It would be nice if we knew what level of CO2, and resulting temperature increase, would bring us to the “point of no return,” that is the point at which positive feedbacks in the climate-glacier system begin to cascade, resulting in catastrophic and irreversible (on a short term) climate change. We don’t know if such a threshold even exists and, if it does, we are unlikely to know it until after we have passed it. Is that tipping point at 500 ppm? 1000 ppm? Or, have we already passed it? We simply do not know.
[text © R. W. Allmendinger (2007). The material reproduced here does not necessarily reflect the view of the Earth & Atmospheric Sciences Department or Cornell University. All figures have been redrafted and re-plotted from the original data sources cited.]