1. Carbon dioxide concentrations in the atmosphere are increasing due to human activity.
Carbon dioxide concentrations are higher now (approximately 380 ppm) than they have been for the past 650,000 years (between 190 to 290 ppm). Human activity has been directly implicated as the cause of this increase by direct measurements of CO2 inputs and a shift in the isotopic composition of CO2 towards a ‘fossil fuel signature’.
2. The concentrations of other greenhouse gases have also increased.
The most striking example of this is methane, the concentrations of which have increased from approximately 700 ppb to 1800 ppb over the past 200 years, despite being below approximately 700 ppb for the previous 650,000 years.
3. Increased CO2 concentrations will cause global warming.
It is important to realise that atmospheric CO2 concentration is not the main control on the Earth’s climate – the sun, albedo (how much incoming light is reflected) and water vapour all have a much greater effect. However, due to the ability of CO2 to absorb infrared radiation, any significant changes in its concentrations alter the energy balance of the Earth and this causes a change in the climate. It is not yet clear what impact the current rate of carbon dioxide release will have on the climate system over the next 100 years, but the uncertainty is related to the magnitude, which could range from 1ºC to 5ºC of warming.
4. The Earth’s climate varies naturally.
Examination of the geologic record indicates that the Earth’s climate has varied widely over billions of years; in fact, some changes are probably more dramatic than human activity will ever achieve. However, those changes generally occurred over millions of years. Even climate variability on shorter timescales appears far less dramatic and much slower than the changes observed in the past 100 years.
5. The climate has warmed by about 1ºC over the past 100 years, which is greater than natural temperature variation over the previous 1,000 years.
This particular point has been the focus of a great deal of debate because warming trends do not perfectly match the change in carbon dioxide concentrations. However, this appears to be due to the multiple controls on the Earth’s temperature, including the influence of volcanoes and solar activity. Those controls are routinely incorporated into climate models but over and above them there seems to be evidence that humans have caused 0.6ºC of warming since the 1960s.
These points are supported by numerous observations but, as with all topics that are truly worth understanding, simplistic explanations rarely reveal the whole truth and our current understanding is incomplete. So with a view to informing the debate, we are currently constructing a list of frequently asked questions regarding climate change that will address these issues in greater detail than can be covered here, or is typically presented in the media. We hope that this will clarify which aspects of global warming are well understood, highlight those that are not, and provide a helpful guide to people interested in understanding how humans affect the Earth’s climate.
This page is available at www.paleo.bristol.ac.uk. We invite you to take a look and give us your comments.