Hasn’t the climate varied through geological time and aren’t the present changes part of that natural variation?
It’s true that 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 that have affected the exchange of carbon between the Earth’s crust and the surface (i.e. the sources and sinks of carbon). The fact that these sources and sinks are rarely in equilibrium causes the amount of the greenhouse gas carbon dioxide in the atmosphere to slowly vary and indirectly affect the global climate.
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 and changes in the tilt of the Earth’s spin axis 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 concentrations of carbon dioxide, methane and nitrous oxide in the atmosphere are the highest they have been for at least the last 800,000 years. Further, the rates of increase in these gases since 1900 appear unprecedented in the last 22,000 years. Computer modelling confirms that global warming during the past 50 years was mainly caused by greenhouse gases released by human activity. In the Northern Hemisphere, 1983–2012 was
likely the warmest 30-year period of the last 1400 years. The average global temperature also appears to be increasing faster with time. As far as we know, what is happening today has no equal in the past.