Some Basics

Content Page from Previous Semesters - For Reference Only

In this module we will not be focusing on the physical climate system or on changes that are occurring to it. Instead we will focus on interactions between climate change and human society. However, a brief understanding of the physical climate is essential to understanding the human-environment interactions.

The Physical Basis Of Climate Change

As a starting point for understanding climate change, we should recognize the difference between climate and weather. The difference is essentially a difference in scale. Climate refers to broad-scale trends in meteorological phenomena such as temperature, precipitation, and wind. Weather refers to local-scale instances of these same phenomena. Whereas climate is often studied in time periods of about 30 years, weather is often studied in time periods of just a few days or even a few hours. Also, whereas weather is very difficult to predict, climate is much more predictable. That is because weather events are driven by highly complex local and regional factors, whereas climate trends are driven by relatively simple processes. For example, we probably don’t know exactly what the temperature will be just two weeks from now in State College, PA (weather), but we can be quite confident that the average temperature in State College will be warmer next July than it will be next January (climate). The process affecting temperatures in different months of the year is simple: tilt in the Earth’s axis of rotation that causes more sunlight to be received during summer than during winter.

Likewise, the primary process driving climate change is also simple: an increase in the concentration of greenhouse gases in the atmosphere. Greenhouse gases cause radiation coming in from the sun to go towards the surface of Earth instead of escaping back into outer space, thereby heating the surface of the planet. As the concentration of several greenhouse gases (in particular carbon dioxide, CO2, and methane, CH4) has increased, more radiation goes towards Earth’s surface, and the planet is heated further. This “global warming” in turn causes other changes in climate, such as changes in precipitation. In this module, we use the term “climate change” instead of the term “global warming” because the overall climatic changes occurring involve more than just temperature, and because the temperature changes are not a uniform warming across the planet (though the average global temperature certainly is warming).

Despite any controversies you might hear about in the news, we are very, very confident that climate change is happening and is caused primarily by human activities. It is true that some non-human processes do change climates, but these processes are either smaller or operate on much longer time scales than the climate change that we’ve been observing recently. The conclusion that humans are causing climate change is based on three very simple, very well understood points:

1. Certain molecules are greenhouse gases.
2. Atmospheric concentrations of some of these greenhouse gases are rising.
3. Global temperatures are rising.

We know (1) from basic physical chemistry and can confirm this in simple laboratory experiments. We know (2) and (3) from both direct observations of the atmosphere and other “proxy” evidence about the atmosphere found in such places as tree rings and ice cores. We further know that (2) is driven mainly by human industrial activity, in particular the burning of fossil fuels, because the change in concentrations coincides with the onset of the Industrial Revolution and because industrial activity is known to emit greenhouse gases into the atmosphere. Finally, we know that (2) is causing (3) due to computer modeling of the global climate system. These models accurately reproduce the observed temperature increases only when the greenhouse gas concentration increases are included. We would literally need to fabricate new physics to explain the observed temperature increases without human-driven increases in atmospheric greenhouse gas concentrations.

The Impacts Of Climate Change

Why do we care about climate change? Because the impacts of climate change on both human and nonhuman systems are quite strong. Unfortunately, these impacts are generally considered to be quite negative (though what exactly we mean by “negative” is a matter of ethics). Impacts include:

• Overall declines in agricultural production, though there may be increases in some places at some times.
• A rise in global sea level, which can in turn force people in coastal regions to migrate.
• Increases in the severity of extreme weather events such as hurricanes and heat waves.

It is important to understand that individual extreme weather events are affected by much more than just climate change. Recalling the difference between climate and weather, we can understand that climate change will affect long-term trends in extreme weather events, whereas individual events are also affected by local factors.

Responses To Climate Change: Mitigation And Adaptation

There are two main ways in which humanity is responding to climate change: mitigation and adaptation. Mitigation refers to efforts to reduce the amount of climate change that will occur via reducing the amount of greenhouse gases in the atmosphere. Adaptation refers to efforts to improve the impacts of whatever climatic changes end up occurring. Exactly what is meant by an “improvement” to the impacts is an ethics question, one which we’ll revisit later in the module as we explore how humanity should arrange its adaptation efforts. Similarly, there are ethics questions in what mitigation efforts humanity should make.

The relationship between climate change, mitigation, and adaptation can be seen in a simple system diagram:

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As this diagram shows, mitigation causes less greenhouse gas emissions, while greenhouse gas emissions cause more climate change. Thus mitigation causes less climate change. Meanwhile, climate change causes more impacts. Climate change can also cause adaptation, which leads to better impacts.