Few scientists deny that human activities can have an effect on local climate or that the sum of such local effects could hypothetically rise to the level of an observable global signal.
The key questions to be answered, however, are whether the human global signal is large enough to be properly measured and if it is, does it represent, or is it likely to become, a dangerous change outside the range of natural variability?
To understand the physical science of climate change, it is important to understand the workings of global climate models and how forcings and feedbacks – and especially solar forcings – affect climate. Scientists can observe climate changes by studying temperature records and making observations of climate change impacts on the cryosphere, hydrosphere and oceans, and extreme weather.
Overwhelming scientific evidence suggests the greenhouse gas-induced global climate signal is so small as to be embedded within the background variability of the natural climate system and is not dangerous. At the same time, global temperature change is occurring, as it always naturally does.