Understanding something as complicated as climate change is really tough. So it’s easy to understand why people don’t always get things right. But it’s much easier to explain why the term “global warming” is misleading than it is to explain why some climate-change messages are only partially understood. So I put the “partial misconceptions” in a separate blog.
Partial Misconception: The greenhouse warming is due to carbon dioxide. Figure 4 shows that slightly over half of the warming near Earth’s surface is caused by carbon dioxide (CO2), with other gases – methane (CH4), Nitric oxide (N2O), halocarbons, and ozone in the lower atmosphere, accounting for the rest of the “forcing.” What is forcing? Forcing can be thought of as a “push” that warms (or cools) the Earth system.
The warming that results is actually larger then you might expect from an increase in these gases alone. This is because the warming surface and air leads to more water vapor, which is also a greenhouse gas. This leads us to the next partial misconception.
Figure 4. Effect of greenhouse gases and aerosols on surface air temperature warming, in terms of “forcings.” From 2007 report, Intergovernmental Panel on Climate Change.
Misconception: Carbon dioxide is the most important greenhouse gas. Certainly this is what you might expect from a first glance of Figure 4. But where is water vapor? I was taught as an Atmospheric Science graduate student that water vapor was the primary greenhouse gas, but carbon dioxide was also important. Modeling studies with various degrees of simplification confirm this first impression. A nice summary can be found on the RealClimate blog.
Why, then, do so many people say that carbon dioxide is the “most important greenhouse gas.” It’s probably because of figures like Figure 4. Note a very important adjective at the bottom which is often ignored, “anthropogenic,” meaning “made by humans.” Humans of course affect water vapor as well, but it cycles through very fast, and the amount of water vapor in the air is basically controlled by the temperature of the air and surface. In a climate model, water vapor continuously adjusts to the conditions within the model, while anthropogenic greenhouse gases in Figure 4 are adjusted by those who run the model.
Put another way, water vapor doesn’t appear in the “forcing” terms for climate models, because it is “internal” to the system. It changes as the result of a “feedback” within the model. Thus external inputs like solar radiation, changes in ground cover, and gases introduced into the atmosphere by human activity are counted as “forcing” but water vapor as not.
In short, we can say that carbon dioxide is the most important greenhouse gas whose amount people are directly altering. Not just in models, but in real life.
Partial Misconception: The warming climate means more exposure to dangerous diseases. I say “partial misconception” because there are multiple factors that change our exposure to disease. Many articles in scientific journals and newspapers discuss increased exposure to malaria, for example, in a warming climate. But that is not the whole story. For example, in the United States, malaria was a real threat over much of the country in the 1700s and the 1800s, and even into the early 20th century. However, public health efforts such as mosquito control and changes in peoples’ habits (for example, using window screens to keep out mosquitoes or staying indoors from dusk to dawn) have largely removed the malaria threat. Similarly, world travel spreads germs, such as the West Nile virus, around the world. This is not a new phenomenon. Europeans coming to the Americas brought small pox with them, leading to the tragic death of countless Native Americans. And populations moving into new areas can expose themselves to new germs.
However, we cannot ignore the fact that vectors for existing diseases will migrate with their preferred climate. Thus at some time in the future, some diseases will show up in areas where they haven’t been before; and in other areas where they have been suppressed.
Partial misconception: The warming climate means more birds will die. Again, there are many factors involved. There are stories of bird populations suffering because food supplies (for example caterpillars) are no longer available when the birds need them, because the two species are responding differently to climate change. However, songbird populations have also suffered because the scarcity of predators like wolves has led to an increase in the number of animals (like raccoons) who eat birds’ eggs. Similarly, pesticides have done serious harm to bird populations. This contributed to a ban on the use of the insecticide DDT in many countries. Finally, the West Nile virus has led to the deaths of many birds (although the magpies and crows, which fell victim to West Nile, seem to be recovering here in Boulder).
Once again, we cannot ignore the impact of climate change. If climate changes continue at the predicted rates, then the entire ecosystem will have to adjust to a new seasonal cycle. This will not be a smooth process: different plants and animals will respond in different ways. And, as in the case of the birds and caterpillars, the food supply will be interrupted at critical times.
Partial Misconception: If we cut back on our production of greenhouse gases, global warming will “go away.” This is true only over a very long period of time. It will take hundreds of years to decrease the carbon dioxide content back to pre-industrial levels through natural processes (the lifetime of carbon dioxide in the atmosphere is around 120 years). This does not mean we shouldn’t consider reducing carbon-dioxide emissions, because continuing the increase in carbon dioxide leads to even more warming than if we slow down the increase in carbon dioxide. One hopeful note is that not all greenhouse gases last as long as carbon dioxide, so reducing their release in the atmosphere might help on shorter time scales. Another hopeful note is that people are studying ways to take carbon dioxide out of the atmosphere, but this is the subject of another blog.
So, when you read or hear about the effects of people on the environment, or try to figure out what you can do to help the environment, please remember that we affect our environments in many ways. Similarly, actions we take to help our environment can improve our environment in many ways. But responding to climate change will remain a challenge for years to come.
