A Climate Minute - The Greenhouse Effect

Greenhouse Gas Levels and Temperature

As far back as scientists can measure CO2 and temperature, it is clear that the two are related. A graph of CO2 and temperature over the past 450,000 years shows just how close this correlation is. Although the relationship between the two is complicated, the result is clear. When CO2 is high, temperatures are high; when CO2 is low, temperatures are low.

A close look at the graph shows that, in the past, rising CO2 has not usually triggered global warming. In fact, during interglacial periods, CO2 begins to rise between 600 and 1,000 years after temperatures rise. Therefore, because a warming period takes about 5,000 years to complete, CO2 may only be responsible for about 4,200 years of that warming. Other factors-a change in solar radiation intensity, Milankovitch cycles, or thermohaline circulation (which varies in different warming periods)-set off rising temperatures initially. Positive feedbacks then bring about a rise in CO2 between 600 and 1,000 years later. These higher CO2 levels cause temperatures to rise even higher. This leads to further positive feedbacks that result in the release of more CO2 and still greater temperatures. Climate models support the idea that greenhouse gases cause about half of the warming that takes place between a glacial and an interglacial period.

Another question is raised by looking at the same figure: Why is CO2 so much higher now than it has been in the past 450,000 years, and yet global temperatures have not climbed high enough to match CO2 levels? The answer to this question is thermal inertia, which is the resistance a substance has to a change in temperature. Thermal inertia explains why even though the Northern Hemisphere receives the most sunlight on the summer solstice, the warmest summertime temperatures don’t arrive until weeks later; or why, even though the Sun is directly overhead at noon, the hottest time of day comes several hours later. Simply put, greenhouse gas levels are rising so rapidly that the temperatures cannot keep up. The lag between increasing green-house gases and the rise of global temperatures appears to be a few decades. The thermal inertia in this case is primarily due to the high heat capacity of the oceans. The models show that global temperature will eventually catch up with greenhouse gas levels, and the full temperature response will be realized.

 Greenhouse Gas Emissions by Country

Not all nations of the world are equally responsible for greenhouse gas emissions and global warming. The United States is the largest emitter of greenhouse gases, followed by the nations China and Russia.

Researchers have been predicting that China will overtake the United States in CO2 emissions by 2010. Some estimates suggest that this already took place in 2006. To calculate per capita emissions, a nation’s total of CO2 emissions is divided by the nation’s population. The top 10 highest per capita emitters in the world are not even in the top 20 nations for total emissions. The reason is that they are small countries with high production and/or low energy efficiency. (Efficiency is the ratio of usable energy output to energy input.) Several of the top 10 per capita emitters are oil-rich nations-including Qatar, the United Arab Emirates, and Kuwait-that do not need to use petroleum efficiently. The nations with high total emissions but low per capita emissions, such as India, have very low productivity for the size of the nation’s population.

The United States is an enormous emitter of CO2, in part because it is the world’s largest economy. But that does not tell the whole story, because the United States emits twice as much CO2 to produce one unit of Gross National Product (GNP) as the nations of Western Europe. This means that the United States is only half as efficient in its energy use as Western Europe. Australia has low total emissions because it has such a low population, but it is nearly as inefficient with energy use as the United States.

Ice core data from the past 600,000 years show that when CO2 is high, global temperatures are high. Although current CO2 levels are higher than they have ever been, temperatures have not caught up with green-house gas levels due to thermal inertia. Scientists say that it is only a matter of time before temperatures rise to match atmospheric CO2 levels. Even with thermal inertia, the hottest years of the last 1,000 years have been in the past two decades, and the numbers of temperature records that have recently been broken indicate that the trend is continuing. Nations vary greatly in the amount of greenhouse gas emissions that they add to the atmosphere, with the United States and China in the lead. Without a doubt, the Earth’s climate future will include the enormous impact that humans make on the atmosphere. Nearly all climate scientists agree that human influence will overshadow natural changes for at least a millennium, until Milankovitch and other natural cycles push the planet toward a new ice age.