The physical basis of climate change refers to our understanding of the physical properties of the climate and how it is changing. In other words, it is the physical (or natural) science behind climate change. Despite being a physical science, it asks some questions of major political and societal importance. Is the climate changing? In what ways is it changing? Are these changes caused by human activity? Because there is so much at stake with the answers to these questions, the physical science of climate change has been the center of extensive attention and a fair amount of controversy. In order to understand the human aspects of climate change, including the political issues, it is very helpful to have some understanding of the physical basis.
Climate vs. Weather
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.
The distinction between climate and weather is very important for understanding climate change. A key point is that the existence of unusually hot or cold weather does not prove or disprove the existence of climate change. Even as the climate changes, there will still be hot days and cold days. Furthermore, just because it is hot or cold where you are does not mean that it is hot or cold worldwide. On any given day, it will probably be unusually cold in some places and unusually hot in others. What matters for climate change are the broad-scale trends. This point is important to keep in mind because people have a tendency to base their beliefs about climate change - perhaps subconsciously - at least in part on their immediate weather conditions. This tendency has been observed in survey research about climate change: when the weather is warmer, more people tend to believe that climate change is happening, and vice versa for colder weather. By recognizing that this tendency exists, we can make conscious efforts to avoid it in our own thinking.
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 stay near 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 timescales 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 surface 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.
Is it because of changes in the sun?
One of the most common arguments made against the physical basis of climate change is that the temperature increases we are observing are due to an increase in incoming radiation from the sun, not due to greenhouse gases from human activity. If greenhouse gases are not changing temperatures, then we don't have much reason to avoid emitting them. But this argument is incorrect. Changes in the sun are not causing temperature increases. The clearest evidence we have for this comes from temperature trends across the atmosphere.
Earth's atmosphere is composed of several layers. The lowest is the troposphere, followed by the stratosphere, mesosphere, thermosphere, and exosphere. If increasing temperatures were due to the sun sending more radiation, then we would expect all of the layers to be warming. However, only the surface and the troposphere are warming. The higher layers have been cooling!
Why would the higher atmosphere layers be cooling? It turns out that greenhouse gases are concentrated in the troposphere. These gases keep more radiation near the surface instead of escaping back towards outer space. With less radiation going from the surface towards outer space, less radiation is passing through the atmosphere layers above the troposphere. So, the temperature increases that we observe on the surface are due to increases in greenhouse gas concentrations, not due to increases in radiation from the sun.
This can be seen from the following figure:
If you would like to read more about this topic, please see the Stanford Solar Center.
The Human Dimensions of Climate Change
The remainder of this module focuses on the human dimensions of climate change, in particular how humans are impacted by climate change and how humans are responding to climate change. 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. 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 systems diagram:
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.