Geologists and anthropogenic climate change

Geologists should know better, but of all the Earth scientists I interact with Economic Geologists seem to be the most skeptical of anthropogenic climate change (ACC). Even though 97% of climatologists agree that humans through, and because of, industrial economy have significantly perturbed the Earth's climate trajectory towards long-term warming <Expert creditably in climate change> Geologists with interests in the mining and petroleum industries are the most vocal of ACC skepticism. I will leave aside the obvious personal motivation for skepticism from this group and focus on their argument. The particular salvo always launched from Geologists of this ilk is that the climate has changed before. The argument is as follows: Climate is always changing. We have had ice ages and warmer periods. Ice ages have occurred in a hundred thousand year cycles for the last 700 thousand years, and there have been previous periods that appear to have been warmer than the present despite CO2 levels being lower than they are now <Resisting climate hysteria>.

It really is quite amazing that these Geologists do not scratch beyond the surface even a tiny bit to see why this may be so. Long before SUVs and coal-fired power plants climate changed naturally in the past, they like to point out, so therefore humans cannot be causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion <Advancing the Science of Climate Change>. To begin to understand this you must ask why climate has changed in the past. Climate changes when it's forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes. There are a number of different forces which can influence the Earth's climate. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, they emit particles into the atmosphere which reflect sunlight, and the planet cools. When there are more greenhouse gases in the atmosphere, the planet warms. These effects are external forces because by changing the planet's energy balance they force climate to change.

It is obviously true that past climate change was caused by natural forces. However, it is just disingenuous, or at best short-sighted, to argue that this means we can't cause climate change. Greenhouse gas increases have caused climate change many times in Earth's history, and we are now adding greenhouse gases to the atmosphere at an increasingly rapid rate. Looking at the past gives us insight into how our climate responds to external forces. Using ice cores, for instance, we can work out the degree of past temperature change, the level of solar activity, and 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 researchers have found, looking at many different periods and timescales 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. We can even quantify this: when you include positive feedbacks, a doubling of CO2 causes a warming of around 3 deg C <Climate Impact of Increasing Atmospheric Carbon Dioxide>.

What does this mean for us today? Rising greenhouse gas levels are an external force, which has caused climate changes many times in Earth's history. 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 greenhouse warming. So past climate change does not tell us that humans can not influence climate; it tells us that climate is highly sensitive to the greenhouse warming we are now causing.