"The climate has always changed" (or similar phrase) does not imply what many seem to think it implies. Let me explain.
Since the 1830s scientists have known that the Earth's climate changes without any human influence, e.g. ice ages. Variations in the Earth's orbit, the location of the continents, the energy given off by the sun & the magnitude of volcanic eruptions can all affect the climate.
Before widespread use of thermometers in the mid-1800s the effects of a changing climate are seen in tree rings, ice cores, corals & other 'natural archives'. The 'little ice age' is the most recent period where natural factors changed the climate by a small but detectable amount
The key point about these natural fluctuations is that they demonstrate that Earth's climate is not stable - it can be altered by small changes in external factors. They are strong evidence that 'climate sensitivity' - the size of change caused by a perturbation - is not small.
We know that human activities have enhanced the natural greenhouse effect and produced a large warming perturbation. The size of observed warming since 1850 is consistent with our knowledge of how much the climate has changed in the past to smaller perturbations.
And we understand the basic physics & chemistry of how human activities influence the climate. Burning fossil fuels & deforestation etc produces carbon dioxide. The absorption of infra-red radiation by carbon dioxide & other greenhouse gases was first quantified in detail in 1861
Since 1850 there has been 1C of warming. The sun's activity increased slightly pre-1950, but then declined while most of the warming occurred. As volcanic eruptions cool the planet, natural factors cannot explain much of the total change & may have even acted to cool the climate.
So, the overwhelming evidence and scientific consensus is that human activities have caused roughly all of the observed warming since pre-industrial times. The good news is that our actions can influence what happens next.
