Climate: The sun’s role is important, but it’s just a cameo
Amid all the new research on how carbon emissions cause global warming, scientists haven’t forgotten about the sun and its role in maintaining the temperature of earth.
Speaking on BBC Radio 4’s Life Scientific programme yesterday, atmospheric physicist Professor Joanna Haigh says the amount of energy given off by the sun can affect earth’s temperature in small but significant way. Relative to the effect of greenhouse gas emissions, however, the sun’s role in climate change is limited, she explains.
We pick out the best bits of her interview with presenter Jim al-Khalili, and look at new research which confirms that fluctuations in the sun’s energy alone can’t explain the accelerated warming trend seen since the 1970s
How does the sun affect climate?
Scientists know that on both long and short time scales, the sun has an important role in controlling the earth’s temperature. Putting it simply, Professor Haigh says:
“Solar radiation heats the surface of the earth – it gets through the atmosphere […] and the surface gets hot and emits heat radiation.”
So it makes sense that subtle variations in the amount of energy given off by the sun can alter the climate. For example, the sun’s energy rises and falls in an 11-year cycle, as this image shows:
Source: NASA
The changes are small, Haigh explains, but significant enough to alter earth’s climate:
“We now know that the radiation varies by perhaps a tenth of one percent over the [11 year] solar cycle. But that’s enough energy to potentially have a small influence on surface temperatures of the earth”
Some climate skeptics suggest these minor fluctuations are the main cause of climate change, as opposed to human produced greenhouse gases.
But Haigh points out that relative to greenhouse gases, the sun’s role in surface temperatures trends from one decade to the next is small. In fact, looking at the “perceptible differences” in earth temperatures caused by fluctuations in the sun actually gives a clearer picture of how greenhouse gases affect the climate, she says.
Humans vs. nature
New research also highlights that while the sun can influence the climate, it’s not the main reason for the temperature rise seen in recent decades. A study published this week shows the amount of sunlight reaching earth’s surface has changed little since the 1970s, yet global air temperatures have continued to rise.
Between the 1940s and the 1970s, less sunlight reached earth surface because particles of sulphur released by industrial processes made clouds reflect more incoming sunlight.
This kept the planet roughly 0.2 degrees Celsius cooler than it would otherwise have been, the study says. Without this effect, temperatures would have risen, but instead they stayed near-constant, creating a 30 year ‘hiatus’ in warming.
Haigh refers to these hiatuses in her Radio 4 appearance, explaining:
“If you look at the temperature record over the past 150 years it wobbles. It doesn’t just go smoothly up with carbon dioxide, it wobbles around all over the place – and there’s been various hiatuses in the past and there’s a hiatus now.”
Scientists believe the current period of slower warming is a result of heat being redistributed from the atmosphere into the deep ocean, since the amount of solar energy reaching the earths surface has remained fairly constant since the 1970s, while greenhouse gases have continued to rise.
Earth responds
Putting the sun’s role in the climate into context, as Haigh does, is important. It’s clear that the planet responds to changes in the amount of energy it receives from the sun. But earth also responds to the rising amount of energy being trapped as humans add more greenhouse gases to the atmosphere.
The research from Haigh and others in that field shows that compared to the effect of greenhouse gas emissions, the role of the sun in temperature trends from one decade to the next is pretty small.