Science says: Climate reacts to whatever forces it to change at the time; humans are now the dominant force changing the climate.
A common skeptic argument is that climate has changed naturally in the past, long before SUVs and coal-fired power plants, so therefore humans cannot be causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. To understand 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.
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 heat-trapping greenhouse gases like carbon dioxide in the atmosphere, the planet warms. These effects are referred to as external forcings 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 forcings. However, to argue that this means we can't cause climate change is like arguing that humans can't start bush fires because in the past they've happened naturally. Greenhouse gas increases have caused climate change many times in Earth's history, and we are now adding greenhouse gases to the atmosphere at a increasingly rapid rate.
Looking at the past gives us insight into how our climate responds to external forcings. 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 we have found, looking at many different periods and timescales in Earth's history, is that when the Earth gains heat, positive feedbacks (vicious cycles) 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°C.
What does that mean for today? Rising greenhouse gas levels are an external forcing, 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 doesn't tell us that humans can't influence climate; on the contrary, it tells us that climate is highly sensitive to the greenhouse warming we're now causing.
Science says: Natural climate change in the past proves that climate is sensitive to an energy imbalance. If the planet accumulates heat, global temperatures will go up. Currently, CO2 is imposing an energy imbalance due to the enhanced greenhouse effect. Past climate change actually provides evidence for our climate's sensitivity to CO2.
If there's one thing that all sides of the climate debate can agree on, it's that climate has changed naturally in the past. Long before industrial times, the planet underwent many warming and cooling periods. This has led some to conclude that if global temperatures changed naturally in the past, long before SUVs and plasma TVs, nature must be the cause of current global warming. This conclusion is the opposite of peer-reviewed science has found.
Our climate is governed by the following principle: when you add more heat to our climate, global temperatures rise. Conversely, when the climate loses heat, temperatures fall. Say the planet is in positive energy imbalance. More energy is coming in than radiating back out to space. This is known as radiative forcing, the change in net energy flow at the top of the atmosphere. When the Earth experiences positive radiative forcing, our climate accumulates heat and global temperature rises (not monotonically, of course, internal variability will add noise to the signal).
How much does temperature change for a given radiative forcing? This is determined by the planet's climate sensitivity. The more sensitive our climate, the greater the change in temperature. The most common way of describing climate sensitivity is the change in global temperature if atmospheric CO2 is doubled. What does this mean? The amount of energy absorbed by CO2 can be calculated using line-by-line radiative transfer codes. These results have been experimentally confirmed by satellite and surface measurements. The radiative forcing from a doubling of CO2 is 3.7 Watts per square meter (W/m2) (IPCC AR4 Section 2.3.1).
So when we talk about climate sensitivity to doubled CO2, we're talking about the change in global temperatures from a radiative forcing of 3.7 Wm-2. This forcing doesn't necessarily have to come from CO2. It can come from any factor that causes an energy imbalance.
How much does it warm if CO2 is doubled? If we lived in a climate with no feedbacks, global temperatures would rise 1.2°C (Lorius 1990). However, our climate has feedbacks, both positive and negative. The strongest positive feedback is water vapor. As temperature rises, so too does the amount of water vapor in the atmosphere. However, water vapor is a greenhouse gas which causes more warming which leads to more water vapor and so on. There are also negative feedbacks - more water vapor causes more clouds which can have both a cooling and warming effect.
What is the net feedback? Climate sensitivity can be calculated from empirical observations. One needs to find a period where we have temperature records and measurements of the various forcings that drove the climate change. Once you have the change in temperature and radiative forcing, climate sensitivity can be calculated. The figure to the left shows a summary of the peer-reviewed studies that have determined climate sensitivity from past periods (Knutti & Hegerl 2008).
There have been many estimates of climate sensitivity based on the instrumental record (the past 150 years). Several studies used the observed surface and ocean warming over the twentieth century and an estimate of the radiative forcing. A variety of methods have been employed - simple or intermediate-complexity models, statistical models or energy balance calculations. Satellite data for the radiation budget have also been analyzed to infer climate sensitivity.
Some recent analyses used the well-observed forcing and response to major volcanic eruptions during the twentieth century. A few studies examined paleoclimate reconstructions from the past millennium or the period around 12,000 years ago when the planet came out of a global ice age (Last Glacial Maximum).
What can we conclude from this? We have a number of independent studies covering a range of periods, studying different aspects of climate and employing various methods of analysis. They all yield a broadly consistent range of climate sensitivity with a most likely value of 3°C for a doubling of CO2.
The combined evidence indicates that the net feedback to radiative forcing is significantly positive. There is no credible line of evidence that yields very high or very low climate sensitivity as a best estimate.
CO2 has caused an accumulation of heat in our climate. The radiative forcing from CO2 is known with high understanding and confirmed by empirical observations. The climate response to this heat build-up is determined by climate sensitivity.
Ironically, when skeptics cite past climate change, they're in fact invoking evidence for strong climate sensitivity and net positive feedback. Higher climate sensitivity means a larger climate response to CO2 forcing. Past climate change actually provides evidence that humans can affect climate now.