Hasn’t the climate varied through geological time and aren’t the present changes part of that natural variation?
The changes, and rates of change, in climate seen today are unprecedented and can only be explained by the effect of mankind’s greenhouse gas emissions.
Climate change has been occurring on Earth for millions of years. For example, the slow movement of tectonic plates and the creation and destruction of mountain chains has caused relatively large changes in temperature and sea level and affected the exchange of carbon between the Earth’s crust and the surface.
The Earth’s climate has also exhibited natural cycles of cold and warm periods which occur over thousands of years due to predictable variations in the Earth's orbit around the Sun which affect the heat reaching the Earth’s surface. In the last few million years there have also been periodic ice ages (up to 10°C cooler than present) and warmer ‘interglacial’ periods (such as the last 11,000 years). During these glacial/interglacial periods, greenhouse gas concentrations and temperature rose and fell together. Regional fluctuations of 1.0-1.5°C have also occurred; for example, northern Europe was cold until the 7th century and temperatures rose 2°C higher than today in the Medieval Warm Period (900-1300 AD).
However, today, the global concentration of carbon dioxide in the atmosphere is the highest it has been for at least the last 650,000 years. Further, the rate of increase in carbon dioxide since 1750 appears unprecedented in more than 10,000 years. Computer modelling confirms that global warming during the past 50 years was mainly caused by greenhouse gases released by mankind. It is likely that the 20th century temperature increase in the northern hemisphere exceeded that of any other century for the last 1000 years. The average global temperature also appears to be increasing faster with time. The Intergovernmental Panel on Climate Change warns of potentially large-scale, abrupt and possibly irreversible changes in physical and biological systems due to this relatively recent and rapid rise in the concentrations of carbon dioxide and other greenhouse gases. As far as we know, what is happening today has no equal in the past.