Hello. I’m Keith Shine. I’m from the Department of Meteorology at the University of Reading. I’m here to talk about climate change. So, climate change could either be a warming or a cooling of the climate– so, for example, some natural climate-change mechanisms, such as large volcanic eruptions and also perhaps variations in the energy emitted by the sun. But there’s also many ways that human activity can cause climate change, by emitting gases or small particles into the atmosphere. Up until now, since the middle of the 19th century the climate has warmed by about .7 degrees centigrade. And as we look into the future, we can expect to see much more climate change as a result of human activity.
Now, predicting the changes into the future is complicated, because we’d need to know things like how the world’s population will grow, about changes in technology, about whether developing nations will catch us up in terms of our consumption of fossil fuels and other resources. So, in climate-change science, we have a number of different scenarios or stories for how the future might change. And in some of them, the climate change by the end of this century might be four, five, six degrees. To others, the climate change would be slower, and maybe two degrees. And that would depend on, for example, moving towards renewable energy rather than burning coal, oil, and gas for our energy.
But that wouldn’t stop climate change straight away, because many of the gases we emit into the atmosphere– for example, carbon dioxide and nitrous oxide– persist in the atmosphere for tens and hundreds of years. So, if we stopped emitting, we would still see those gases in the atmosphere in the future. And they would only slowly decay. So, even if we were to change our emissions today, we would still see an effect of our activity, for example, at the end of this century. So, unlike temperature, where we would expect to see a warming almost everywhere, rainfall in some areas will decrease, and rainfall in some areas would increase.
And we expect that areas that are already wet– for example, the tropics and the mid latitudes– will get wetter, and those areas that are already dry, like the desert zones, will get even drier. We might expect the most intense hurricanes to get more intense. And the very heavy rainfall events that often lead to flooding, we would expect those to get more intense or more common as we go into a changing climate. The most important greenhouse gas in the earth’s atmosphere is water vapour. And that’s there naturally. It results from evaporation from the sea surface and land surfaces. And it’s removed by rainfall. But the main effect of human activity is to increase the concentrations of carbon dioxide.
So it’s the main greenhouse gas due to human activity. And that’s emitted mostly from burning of coal, oil, and gas but also from changes in land use. The second most important greenhouse gas from human activity is methane. So, methane is emitted largely by agricultural systems– for example, enteric fermentation in cows and other livestock and also from manure production. All natural gas, for example, is methane, so any leakage in natural gas between it being drilled out of the ground and it reaching your home is also adding methane to the atmosphere. The third most important gas is nitrogen oxide, which is also emitted from many agricultural sources– for example, soil management. And it’s a rather long-lived greenhouse gas.
It persists in the atmosphere for 100 years before it’s slowly broken down by chemical reactions in the atmosphere. Now, there are a few human activities that, instead of enhancing the greenhouse effect and causing a warming, actually contribute to a cooling. Tiny particles that we emit into the atmosphere– for example, from burning coal and oil– leads to tiny particles of sulfuric acid and related substances. And also burning of forests also leads to tiny particles getting in the atmosphere. And they reflect the sun’s light and lead to a cooling effect. But the net effect of human activity, the increase in greenhouse gases, is, as we believe, much more important. And therefore the overall effect of human activity is a warming.
Detecting climate change isn’t the same as attributing that climate change to a cause. And so we use our best understanding to understand what has caused the climate to change. And certainly the climate change that we’ve seen since the mid 20th century, from about 1950 onwards, is mostly due to human activity and mostly due to the greenhouse gases like CO2 that we’ve emitted into the atmosphere. In the period before 1950, when the climate change was smaller, then it was probably a mixture of natural climate change and human-induced climate change.