We all want the climate to not change. But it will, and it always has. It is important to clearly understand what is meant by climate. Climate is the average weather over an extended period of time. Climate change can be for almost any length of time. Often in Meteorology it is 30 years, but for this discussion, climate is defined to be hundreds to a thousand years.
All the yearly and decadal variations go on top of the climate trend. And these higher frequency variations can be as large or larger than the climate trend, necessitating many years of data to get an accurate estimate of the long term change in the weather. Climate can also be regional or global.
There are two key elements used in assessing climate change. The first is to look at the (global) change in weather over many, many years, and perhaps extrapolating any trend into the future. The second is a climate model that would calculate the future climate much as weather forecasters uses a model to forecast the weather for the upcoming week. So, data and models are the back-bone of climate research and prediction. Data may be in the form of long term records of data from cities scattered around the globe. It can be tree rings, or ice cores where the deuterium to hydrogen ratio can be measured to obtain temperature.
There have always been large changes in global climate. The extremes in temperature in the earth’s history have been absolutely huge, ranging form a mile thick ice covering everywhere, to no ice at all. This is all before humans existed. Many of warming periods were associated with an increase in carbon dioxide. Decreasing amounts of carbon dioxide accompanied periods of cooling. The carbon dioxide concentration is highly correlated with the temperature. But a high correlation does not imply causality or the lack thereof. Other extremes in temperature are related to earth’s periodic orbital changes.
It is useful to remember a demonstration used in a blog some time ago, where the level of water in a tank was a proxy for temperature. A hose at the top poured in water and represented the energy coming from sunlight in the visible part of the spectrum. A spigot at the bottom represents the energy escaping to space in the invisible infra red (IR) portion of the spectrum. When the amount leaving equals the amount coming in the water level (temperature) stays the same. At night there is no sunlight coming in so the spigot drains out water and the water level (temperature) drops. That is why it is colder at night. During the day, the effect is reversed. The same is true for the atmosphere. If the spigot gets clogged, it corresponds to an increase in absorbing trace gasses, and less of the energy the earth radiates to space can get out, just as the same energy is trapped inside a car on sunny summer day.
When we talk about greenhouse gasses, the preponderance of the conversation centers on carbon dioxide, and the carbon dioxide that is a by-product of combustion of fossil fuels: natural gas, coal or oil. Yet there are many other radiative active trace gasses. Some of the most active and their concentrations in the atmosphere are: carbon dioxide (CO2) 0.04%, water (H2O) up to 4%, Methane (CH4) 0.0002%, Ozone (O3) 0.001%. We want more Ozone to protect us from UV radiation, yet the more Ozone there is the greater the greenhouse warming will be. We want more water to produce fresh water, but the more water in the atmosphere to create rain, the hotter the temperature.
There was a time in the last 2000 years of recorded history that we had very cold periods and very warm period. There was no anthropogenic cause because there simply enough people to make a difference and also it wasn’t until the industrial age that the wide spread use fossil fuels began to increase the concentrations of carbon dioxide. Now, for the first time in earth’s history, we can change the natural course of events. And that is what the Global Warming debate is all about.
Next time we will examine what the data shows for global warming.