23 8.3 Anthropocentric Climate Change
Weather and Climate
When it comes to defining climate, it is often said that “climate is what you expect; weather is what you get.” That is to say; climate is the statistically-averaged behavior of the weather. In reality, it is a bit more complicated than that, as climate involves not just the atmosphere, but the behavior of the entire climate system—the complex system defined by the coupling of the atmosphere, oceans, ice sheets, and biosphere. Weather is the current conditions of the atmosphere for a specific location and time.
Having defined climate, we can begin to define what climate change means. While the notion of climate is based on some statistical average of the behavior of the atmosphere and ocean, this typical behavior can change over time. That is to say, what you “expect” of the weather is not always the same. For example, during El Niño years, we expect it to be wetter in the winter in California and snowier in the southeastern U.S., and we expect fewer tropical storms to form in the Atlantic during the hurricane season. So, the climate itself varies over time.
If the climate is always changing, then is climate change by definition always occurring? Yes and No. A hundred million years ago, during the early part of the Cretaceous period, dinosaurs roamed a world that was almost certainly warmer than today. The geological evidence suggests, for example, that there was no ice even at the North and South poles. Climate change is a naturally occurring process of the planet, following a variety of different cycles. Something else is occurring that is causing the planet to warm
So, the significant climate changes in Earth’s geologic past were closely tied to changes in the greenhouse effect. Those changes were natural. The changes in greenhouse gas concentrations that scientists talk about today are, however, not natural. They are due to human activity.
The scientific consensus demonstrates that climate change in the 21st century is necessarily a human problem. People are causing climate change through their everyday actions and the socioeconomic forces underlying those actions. At the same time, people are feeling the consequences of climate change through various impacts on things they value, and through the responses, they are making to address climate change.
Climate is the average of weather (typically precipitation and temperature) in a particular location over a long period, usually for at least 30 years. A location’s climate can be described by its air temperature, humidity, wind speed and direction, and the type, quantity, and frequency of precipitation. Climate can change, but only over long periods. The climate of a region depends on its position relative to many things.
Scientific Consensus
The scientific consensus is clear, in that 97 percent of all scientists who directly study climates and climate change believe that the current warming of the planet is anthropogenic (human) in nature