The significance of past climate change

A common skeptic argument is that climate has changed naturally in the past therefore humans aren't causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. When I try to explain why to people, I usually get blank, confused stares. I gave a presentation to a roomful of engineers this week and after explaining the significance of past climate change, complete with slides of climate sensitivity PDFs and examples of positive feedbacks, the result was a long, silent pause. I asked if anyone understood what I'd just talked about. A few asked some follow-up questions which made it clear they didn't. So I'm reworking my whole explanation of past climate change in an attempt to make it as clear and simple as possible. Comments, particularly on anything confusing or unclear, are welcome!

In the past, climate has changed, sometimes very dramatically. This has gone on long before SUVs and coal fired power plants. If climate can change on its own, couldn't current global warming be natural as well? To answer this, first you have to ask why climate has changed in the past. It doesn't happen by magic. Climate changes when it’s forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes.

This can happen in a number of ways. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, all the particles suspended in the atmosphere reflect sunlight and the planet cools. These effects are referred to as external forcings because by changing the planet's energy balance, they force climate to change.

Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, we can work out past temperature change, the level of solar activity plus the amount of greenhouse gases and volcanic dust in the atmosphere. From this, we can determine how temperature has changed due to past energy imbalances. What we have found, looking at many different periods in Earth's history, is that when the Earth gains heat, positive feedbacks amplify the warming. This is why we've experienced such dramatic changes in temperature in the past. Our climate is highly sensitive to changes in heat.

What does that mean for today? Rising CO2 levels are an external forcing. They're causing an energy imbalance and the planet is building up heat. From Earth's history, we know that positive feedbacks will amplify the CO2 warming. So past climate change doesn't tell us that humans can't influence climate. On the contrary, the past tells us that climate is highly sensitive to the CO2 warming we're now causing.

Posted by John Cook on Wednesday, 21 April, 2010

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