How do we know the world is getting warmer?
Our planet has been warming rapidly since the dawn of the Industrial Revolution.
The
average temperature at the Earth's surface has risen about 1.1C since 1850. Furthermore, each of the last four decades has been warmer than any that preceded it, since the middle of the 19th Century.
These conclusions come from analyses of millions of measurements gathered in different parts of the world. The temperature readings are collected by
weather stations on land, on ships and by satellites.
Multiple independent teams of scientists have reached the same result - a spike in temperatures coinciding with the onset of the industrial era.
Scientists can reconstruct temperature fluctuations even further back in time.
Tree rings, ice cores, lake sediments and corals all record a signature of the past climate.
This provides much-needed context to the current phase of warming. In fact, scientists estimate the Earth hasn't been this hot for about 125,000 years.
How do we know humans are responsible for global warming?
Greenhouse gases - which trap the Sun's heat - are the crucial link between temperature rise and human activities. The most important is carbon dioxide (CO2), because of its abundance in the atmosphere.
We can also tell it's CO2 trapping the Sun's energy. Satellites show less heat from the Earth escaping into space at precisely the wavelengths at which CO2 absorbs radiated energy.
Burning fossil fuels and chopping down trees lead to the release of this greenhouse gas. Both activities exploded after the 19th Century, so it's unsurprising that atmospheric CO2 increased over the same period.
