There is strong evidence that the climate change we are experiencing today is very likely due to the greenhouse gases emitted by humankind. However, when we look back thousands or even millions of years, we can see that the climate has changed dramatically in the past, well before humans were around. Some of these changes even caused mass extinctions. What's more, year after year, there are smaller fluctuations in our climate that would happen even if humans weren’t around. These fluctuations are often called ‘natural climate variability’ and are caused by a number of different influences.
Changes in solar activity
Scientists know that the amount of energy the Sun emits varies on a cycle of about 11 years and it is thought that some of the variations in our weather might be linked to this cycle. Most climate scientists agree, however, that the changes we’ve observed in the solar cycle are not large enough to fully account for the rapid temperature rises over the last 50 years.
Changes in Earth’s orbit
The way the Earth moves around the Sun and how it moves on its axis aren’t constant through time. There are cycles in Earth’s orbit and tilt that can have a dramatic effect on our climate, instigating ice ages or changing seasonality. However, these variations, known as Milankovitch cycles, last tens to hundreds of thousands of years, too long to account for the changes in climate we have experienced over the last 50 years.
We know volcanoes emit two gases which can have an impact on global temperatures — sulphur dioxide (SO2) and carbon dioxide (CO2). They each have very different effects and work on different timescales:
- SO2 — when this gas is emitted to high altitudes (about 50,000 ft or above) it enters the stratosphere. Here it can form acid droplets that partially scatter and reflect sunlight away from the Earth, cooling the surface. The droplets have a fairly immediate impact and, if there are enough of them they may cool the climate for a few months — or even a year or two — but then the droplets fall out of the stratosphere and things return to normal.
- CO2 — we know this is a greenhouse gas, so when it is emitted in large enough quantities it can have a warming effect on our climate. But the amount of CO2 emitted from volcanoes isn’t changing much over time and is also very small in comparison to the amount emitted by humankind’s activities. .
Atmospheric aerosols are microscopic particles emitted from human and natural sources that become suspended in the atmosphere. Aerosols are produced naturally from volcanoes and forest fires, as well as by humans from fossil fuel power stations and other industrial activities.
Many aerosols cool the climate by scattering and absorbing sunlight and also by affecting clouds. In fact, it is likely that some of these man-made pollutants have off-set some of the global warming we might have otherwise experienced. Despite this cooling effect, the net impact of human activities, combining greenhouse gases and aerosols, has been to warm the world's climate.
El Niño and La Niña
In the late 1800s Peruvian fishermen named a seasonal warming of the Pacific Ocean El Niño (Spanish for ‘little boy’). El Niño takes place on average every five years and can last up to 18 months or more.
It occurs when warmer-than-usual ocean water pools off the west coast of South America. During La Niña (Spanish for ‘little girl’) this is reversed and the surface waters in the eastern Pacific are cooler than normal. After the seasons, El Niño and La Niña are the single largest cause of year to year climate variability. Their impacts are felt around the world.