Climate Change and the Oceans

Man's Impact on our Climate

As we have previously shown, out climate IS changing. The temperatures of the earth and oceans are increasing, sea ice and glaciers are decreasing. The next question is: What is causing this to happen? Is this change natural? Or: Is Man causing the climate to change?

We present here some evidence that Man is causing the climate to change.

Carbon Dioxide Levels

This graph shows the steady increase of atmospheric CO2 concentration from about 1855, near the start of the industrial revolution, to the present time. The increase parallels what we would expect to see if the increase were due entirely to anthropogenic (human) activity, and this will be touched on in later figures.
Note that levels of CO2 concentration exceeding about 300 ppm are not seen in the last half million years (see "Sudden Regime Shifts" and Vostok ice core record). That is, CO2 is now at a level that is higher than in the warmest interglacial periods for the last 4 ice age cycles. Thus, even this long record cannot indicate to us how global temperature might respond to these high levels, which are predicted to continue to increase.

The recent atmospheric CO2 increase appears even more dramatic and clearly aberrant when compared to conditions over the past millennium. The figure to the right shows the CO2 concentration in Antarctic ice cores for the past millennium (Siegenthaler et al., 1988; Neftel et al., 1994; Barnola et al., 1995; Etheridge et al., 1996). Recent atmospheric measurements at Mauna Loa (Keeling and Whorf, 2000) are shown for comparison.

Increase in Other Greenhouse Gases

(a) Changes in the atmospheric concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over the past 1000 years.
The figures to the left and below show the human influence on the atmosphere during the Industrial era. The CO2 plot duplicates the previous one. Methane is extremely effective as a greenhouse gas and has increased in pace with CO2. NO has also increased. The recent increases in concentration of these gases and aerosols are due to fossil fuel use (which adds CO2, NO and sulphate to the stmosphere), deforestation (burning adds CO2 and loss of forests prevents its useage in photosynthesis), and agriculture (CO2 and NO generated in fertilizer manufacture, and methane generated by waste decay).

b)time history of the concentrations of sulphate from ice cores in Greenland.

From these results it is clear that human activities are changing the composition of the atmosphere, increasing the concentration and changing the mix of greenhouse gases. From first principles (see "The Basics - Radiation Balance & Greenhouse Gases") we would expect this to lead to a rise in the earth's temperature. However, there are many other processes that might occur as temperature tries to increase. Some of these are examples of "negative feedback", where the process resists the temperature change. For example, increased cloudiness might lower the fraction of solar radiation that ever reaches the lower atmosphere. Other processes, however, cause positive feedbacks. For example, reduced sea ice increases absorption of solar radiation by the ocean. Positive feedback processes might, in extreme cases, cause runaway warming.

There are a couple of ways to decide whether the net effect of all the complex interactions in the climate system will cause increased CO2 concentration to increase global temperature. We can look at the historical record over a period that goes back beyond the start of the rapid rise in CO2, and we can build sophisticated numerical models that attempt to model every significant process that occurs in the global climate system.

This plot shows the northern hemisphere temperatures over the past 1000 years as based on climate proxies (dark blue) and thermometer based recording (light blue). CO2 concentrations (red) are those recorded in the Law Dome (Antarctic) ice core and at the Mauna Loa monitoring station in Hawaii. Temperature data are those of Mann et al, 1999. Law Dome ice data are available from the Paleoclimatology Branch of the National Climatic Data Center, and Mauna Loa data are available from the Carbon Dioxide Analysis Center.

As you can see, temperature rise closely mimics CO2 levels. Of course, it is a lot more complicated than this. There are feedback mechanisms involved as well as natural variations in climate, but at a very basic level, we know that man is influencing the climate.

Models Validate Human Influence

Models have been used to assess man's impact on global surface temperatures.

Panel (a) compares observed temperatures over the past century with the output from a climate model that has been run without including the anthropogenic greenhouse effect. Panel (b) compares observed temperatures with output from a model that incorporates only the greenhouse effect. Panel (c) makes the same comparison only with model results that incorporate both greenhouse and natural forcing. (see Intergovernmental Panel on Climate Change (IPCC), 2001: Climate Change 2001: Synthesis Report)

You can see from this figure that current temperature variations can only be explained in the modeling experiments that included anthropogenic forcing. Although natural variations account for some of the changes in climate, they do not account for everything. We are having an impact on our world.