If we can’t predict weather two weeks ahead, how can we predict climate fifty years from now?
Climate refers to the average environmental conditions in a particular place over time, usually over a 30-year span. Weather describes the actual conditions at a given time, day-to-day, and even hour-to-hour.
There are plenty of other familiar situations where the same sort of difference applies. For example, we know the average lifespan of people born in the US, but individuals live a lot longer or a lot shorter than the average, depending on their diets, lifestyles, genetics, and other factors.
The difference between average conditions and actual conditions is why weather reports include two numbers — the normal, or average, high for the day, which is climate; and the actual high for the day, which is weather, or at least one aspect of weather.
The reason it’s so hard to predict the weather very far in advance is because weather is incredibly complex and dynamic. Factors like today’s temperature, humidity, prevailing winds, and local geography all have an influence on tomorrow’s weather. What happens tomorrow determines what will happen the next day, and so on. Even the tiniest unknown factor in today’s weather, say the humidity over a patch of forest, increases the uncertainty of making tomorrow’s forecast. The situation makes the forecasts for next week even less certain, and forecasting even further into the future becomes increasingly more challenging.
Climate is very different. While climate is also an exceedingly complex system, we aren’t looking at local, day-to-day details, but rather focusing on the average conditions for a region over time. And those change much more slowly. Nobody can tell you what the temperature will be on August 10, 2020, in New York City, but it’s very likely to be a lot warmer than on February 10, 2020, because New York City’s climate is hot in summer and cold in winter. The city also happens to have a fair amount of both rain and snow. The climate in San Diego, California, is warm, but not hot most of the year, with relatively little rain and essentially no snow. Buffalo, New York, is very cold in winter and gets a huge amount of snow. And so on. You instinctively know the climate where you live because it doesn’t change a lot from year to year.
There are many factors that determine climate, including the Earth’s orbit around the Sun and the Sun’s brightness. Climate is also governed by changes in ice sheets, in ocean currents, and in the gases that make up the atmosphere. Climate scientists continue to use Earth observations and models to improve their understanding of these forces. They now have decades of environmental data and model experience to know in a general way what’s likely to happen if one of those factors changes. As a consequence, it is possible to project with some confidence — while acknowledging the uncertainties — what the climate will be like 50 years in the future based on general trends; on computer models that capture, though not perfectly, the dynamics of the climate system; and on our scientific insights into past climate conditions. The dark blue band (all years) and the light blue line show how rarely the actual temperature hits the average.