Climate Change Lesson 5: Climate Forcing and Uncertainty

It is important for students to understand that the vast bulk of scientific data and analysis undertaken over decades by thousands of people support the proposition that climate is changing, that climate is, in general, warming, and human activity is the most likely cause. Major models of climate change, while generating different predictions tend to agree in the direction of change. The governments of most countries in the world, evidenced by their support of the Kyoto accord, recognize that climate change is real and a major problem, even if there is substantial disagreement on actions to be taken, and who and how, to pay for them.

It is also important for students to understand that while most scientists agree, a minority are skeptical. Climate modeling is extraordinarily complex, and predictions of the future are never certain. The history of science is filled with examples of the majority scientific opinion not being the correct opinion. It is equally full of attempts to debunk good science when it is not in people’s economic interests.

Probabilistic thinking is an especially important aspect of climate modeling. This lesson, and the lesson on modeling that follows, are attempts to be clear about the complexity of the process. The lesson on forcings shows how scientists try to build uncertainty into their models, and helps students recognize potential sources for error.

Environmental observations are the foundation for understanding the climate system. From the bottom of the ocean to the surface of the Sun, instruments on weather stations, buoys, satellites, and other platforms all collect climate data. Data from observations along with experiments and theory are used to construct and refine computer models that represent the climate system and make predictions about its future behavior.

Models are tested by predicting the past and the present. If models can be created that can predict past temperature changes based on other data we can have confidence, but not certainty, that these models can help predict the future.

This lesson will introduce students to the major factors (positive and negative forcings) which are part of climate models, along with three factors which make forecasting the future of climate change challenging.

First, positive (heating) forcing and negative (cooling) forcings are difficult to predict. With each forcing, scientists try to estimate both the most likely result, and the range of estimates, from the highest to lowest.

Second, time lags add to the difficulty of making predictions. Forcings don’t change the global average surface temperature instantaneously. It may take years or even decades for the full impact of a forcing to be felt.

And third, forcing can influence other factors (feedback loops). CO2 can result in a rise in temperature which can increase evaporation meaning more cloud cover which would increase reflection of solar energy and result in cooling.