We should distinguish at the outset between climate and weather. Weather is chaotic, which means that it cannot be forecast very far into the future. Small errors in the forecast grow with time, until eventually the forecast is nothing but an error. By climate, we mean some sort of an average of the weather, say averaged over 10 years, more or less. We cannot predict the details of rain versus shine on Tuesdays versus Saturdays very far into the future, but we can hope to forecast the average rainfall in some location at some time of year. Weather is chaotic, but by taking the average, we arrive at something that is not chaotic, which seems to be in some ways predictable. We will return to this topic in Chapter 6.
Human-induced changes in climate are expected to be small when compared with the variability associated with weather. Temperature in the coming century is projected to rise by a few degrees centigrade (Chapter 12). This is pretty small compared with the temperature differences between the equator and the poles, between winter and summer, or even between day and night. One issue this raises is that it is trickier to discern a change in the average when the variability is so much greater than the trend. Careers are spent computing the global average temperature trend from the 100+ year thermometer record (Chapter 11).
A small change in the average relative to a huge variability also raises the question of whether a change in the average will even be noticeable. One way that the average weather matters is in precipitation. Ground water tends to accumulate, reflecting rainfall over the past weeks and months and years. It may not matter to a farmer whether it rains on a Tuesday or Saturday, but if the average rainfall in a region changes that could spell the difference between productive and nonproductive farming. A change in the average climate will change the growing season, the frequency of extreme hot events, the distribution of snow and ice, the optimum growth localities of plants and agriculture, and the intensity of storms.
In addition to day-to-day weather, there are longer-lasting variations in climate. One past climate regime was the Little Ice Age, approximately 1650-1800, bringing variable weather, by some records 1°C colder on average, to Europe. Before that was the Medieval Optimum, perhaps 0.5°C warmer over Europe, coincident with a prolonged drought in the American southwest. We will discuss the causes of these climate changes in Chapter 11, but for now it is enough to observe that relatively small-sounding average-temperature shifts produced noticeable changes in human welfare and the evolution of history. The climate of the Last Glacial Maximum, 20,000 years ago, was so different from today that the difference would be obvious even from space, and yet the average temperature difference between then and today is only about 5-6°C (Chapter 8). Another implication of these natural climate shifts is that it makes it more difficult to figure out whether the present-day warming is natural or caused by rising greenhouse gas concentrations and other human impacts on climate.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.