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March 11 2014
Sun not driving changing climate
This is a post about statistics. Please don’t run away; understanding what this new research says about the sun and the Earth’s climate needs it and I hope it’ll make some sense.
The sun has done some interesting things over the past 1000+ years. There have been periods with a lot of sunspots and periods of limited activity, including the Maunder minimum –from 1645 to 1715 a.d. when sunspots became very rare. When we’re thinking about climate change, these periods are important because the amount of energy the sun puts out varies a tiny bit, by 0.1 to 0.25%, as the number of sunspots changes.
It has been noted that some of those time periods, like the Maunder minimum…the Earth also was also cold. Part of the “little ice age”, a time from about 1500 to 1800 a.d. when the Earth was colder than average, overlaps with this solar minimum. That overlap suggests that the sun’s activity drop could have caused the little ice age…but did it? That’s where the statistics in this newly-published science comes in.
As you have probably heard, correlation doesn’t equal causation…the fact that the sun was a tiny bit less active during the little ice age doesn’t mean it caused it. Scientists from the University of Edinburgh set up a statistical problem to test this hypothesis.
There are detailed records of how the Earth’s temperature has varied over the past several thousand years constructed from things like tree rings, ice cores, and ocean temperature measurements. There are also records of other influences on the climate, such as volcanoes and greenhouse gases emitted by mankind.
Each of these forces influences the climate and statistics can tell us which one matters most. For example…if the sun was causing the little ice age, then other times when the sun was hotter should also fit with times the Earth was hotter. After testing the data, the scientists found…this just wasn’t true.
Over the past 1000 years, the sun’s influence on climate is just tiny. When the sun warms up by 0.1%, it just isn’t all that much of a change compared to other influences. On the other hand, 2 effects correlate strongly with climate; volcanic eruptions and greenhouse gases.
When volcanoes erupt they send ash into the atmosphere which can reflect sunlight and cool the planet, while greenhouse gases act to keep sunlight in. The little ice age was a period when more volcanoes erupted, and that correlates very strongly with the temperature change. When the volcanoes slowed down, the planet warmed; the volcanoes were the main driver, not the sun. Meanwhile, the temperature of the planet has warmed by about 1°C over the last century and the only thing which correlates with that warming is increases in greenhouse gases.
This research signals that there’s no “get out of jail free” card for climate change. The sun hasn’t changed much over the last 1000 years, and the most it ever changes accounts for less than 10% of what humans have done in the last century alone. Even if the sun went into another minimum starting right now, it wouldn’t put a dent in the changes humankind has caused.
For the purposes of the planet’s climate, the sun is there, it supplies the energy, but it just doesn’t change all that much over short times. When the Earth warms up or cools down, it isn’t happening because of big changes on the sun; it’s happening because of big changes on the Earth, and right now the biggest change happening on the Earth is the buildup of greenhouse gases in the atmosphere from human activity.
Image credit: NASA/SDO (look for Venus!)
Cosmos: A Spacetime Odyssey
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