At 4.52 billion years of age, our planet is no spring chicken.
While still young compared with the age of the universe (about 13.7 billion years), it has seen and experienced a lot of weather and climate. One big climate-changing event about 65.5 million years ago occurred when a space rock, about 6 miles in diameter, and traveling an estimated 15 miles per second, slammed into Earth's surface in the Yucatan in northeast Mexico.
Its mass and kinetic energy was so great that it plowed miles deep into the Earth's mantle creating a crater that is about 120 miles wide. Then things got really interesting. The underlying carbonate and sulfate rocks released hundreds of billions of tons of water vapor, carbon dioxide and sulfur dioxide gases. An enormous plume of these gases along with ash, dust, rock, boulders and all manner of debris erupted into the sky and was globally distributed with huge consequences. At least it was for the dinosaurs. It is believed they all died in a short period of time.
This event is an excellent definition of "climate forcing." This climate-forcing event caused fires to burn for a long time, and along with the ash, hid the sun. More than 50 percent of the land and ocean species disappear from the fossil record. A recent paper in Science (Vol. 327, 5 March 2010, p1214) with 33 authors reaffirm these events. Many other sudden and not-so-sudden forcing events have occurred during Earth's long history.
A not-so-sudden climate-forcing event discussed in this column previously is the formation, and subsequent melting, of huge glaciers during the many ice ages that have occurred in the past. The Earth's orbital cycles detailed in the Milankovich theory are believed to be responsible for these events. They take place over thousands and tens of thousands of years. In a way, these two natural episodes mark the natural bookends of climate change from the very sudden to the lengthy.
Now let's talk about our current era. Some geologists are calling this period we live in today the "Anthropocene" because of man's influence on nature and everything around us. The data available from a myriad of sources is quite clear: global warming is happening, now. The IPCC 2007 report on page 3 states with "very high confidence that the global average net effect of human activities since 1750 has been one of warming." This terminology means that there is at least a 9 out of 10 chance this statement is correct. Many people are now being persuaded that indeed this may be true: others think the warming may be just part of a "normal" cycle.
Let us see what the science has to say. There are several different components known to influence climate. A component of the atmosphere that has a cooling effect causes a negative forcing (below zero), while a component that causes warming is a positive forcing (above zero).
There is a consensus among climate scientists as to the impacts on climate of the different components of the atmosphere that exist today based on changes from 1750, the start of the Industrial Revolution. Long-lived greenhouse gases (GHGs), carbon dioxide, methane, nitrous oxide and halocarbons are all positive, or warming, forcing agents. Ozone, which at high altitudes (stratospheric) has a negative or cooling effect, whereas ozone in the lower atmosphere (troposphere) has a positive or warming effect. High-altitude water vapor has a small but positive warming effect.
Others include land use, such as clear cutting (cooling); black carbon on snow (warming); aerosols, such as water droplets, sulfur oxides and light-colored particles from volcanic eruptions (both are strongly cooling — they reflect solar radiation back into space); contrails left by high flying planes (very slight warming); and solar irradiance, or changes in the energy intensity of the Sun (warming).
When we add these all up, we have a net anthropogenic component, which is positive and therefore strongly forcing (warming), and the increased level of carbon dioxide is the main contributor.
Now let's take a look at what Earth Observatory, a part of NASA, has to say. They recently summarized in three curves all of this data and more. The dashed line is the "measured" (instrumental) temperature data for the last 105 years; the lower solid line is the "no human influence" global temperature based on natural forcings (such as the four volcanic events shown). Using a variety of climate models, the upper solid curve is the expected temperature based on "human influence." The agreement between the "measured" and the "human influence" lines is excellent. This is very strong support that the increased global temperature anomaly seen is due to human influences. The global warming we are seeing today is not "natural."
The ability of models to generate reasonable histories of global temperature is verified by their response to four 20th-century volcanic eruptions. Each eruption caused brief cooling that appeared in observed as well as modeled records. (Graph adapted from Hegerl and Zwiers et al., 2007.)
In the news today, we see particles blown into the atmosphere by a volcano in Iceland and feel distressed about the air pollution we can see. Or, we see oil on the sea surface caused by a drilling accident in the Gulf. In the case of the oil spill we can see it, and if close enough, we could also smell it, touch it and see its effects on oil-soaked birds and dead turtles. It evokes a very visceral response.
However, if we burned the 5,000 to 25,000 gallons of leaking oil (estimate) released into the ocean every day in our car or home we would think nothing of it. We can't see the resulting carbon dioxide, can't smell it, can't taste or touch it. We fill up our gas tank again and go on our way. And yet every day the United States consumes over 1 billion gallons of oil, turning it largely into carbon dioxide and water vapor, and it evokes no public response or awareness. We can't sense this gas; and that is the perception problem we have. Pollution is not pollution if one can't sense it. This, in part, makes it very hard for the public to recognize that GHGs are causing global warming and that these need to be dealt with.
The scientific career of Raymond N. Johnson, Ph.D., spanned 30 years in research and development as an organic/analytical chemist; he is currently founder and director of the Institute of Climate Studies USA (www.ICSUSA.org). Climate Science is published the first Sunday of every month.