The science of global warming

This page sets out the basics of global warming. Why it is happening. How we know.

It all starts with the sun

The sun’s radiation powers life on earth. Without it, earth would be a sterile, dead planet.

Greenhouse gases, which include carbon dioxide (CO2, the most important), water vapour, methane, nitrous oxide, ozone and other chemicals such as chlorofluorocarbons (CFCs), surround the earth.

Short wave solar radiation from the sun reaches the earth’s surface and some is reflected back as long wave radiation. The greenhouse gases trap some of this reflected heat, in the same way that glass traps heat inside a greenhouse.

This keeps the planet warm – 33 degrees warmer than it would be without them – and habitable.

How the sun's radiation is absorbed and reflected.
How the sun’s radiation is absorbed and reflected. (Think Green, not Greenhouse). I am still seeking permission to use this.

Gas concentrations are increasing

Unfortunately, the concentration of greenhouse gases has increased in the past 200 years.

Greenhouse gas concentrations for past 2000 years (American Chemical Society, from IPCC Fourth Assessment Report (2007)). Recent values measured directly, historic values obtained from ice cores.

For example, carbon dioxide concentrations have increased from 300 parts per million to 400 ppm in that time. This has raised the air and water temperatures by about 1 degree so far.

Atmospheric Carbon Dioxide over the past half century.
Atmospheric Carbon Dioxide over the past half century. (NOAA)

So the world has become hotter, and temperatures are continuing to rise. The last 6 years (2014-2019) have been the six hottest years on record since 1880.

Global temperatures from 1880 to now.
Global temperature from 1880 until now (NASA)

Has this ever happened before?

Scientists can estimate past temperatures and carbon dioxide levels using ice cores (which trap air inside), tree rings, the composition of rocks, and other methods. These estimates indicate that CO2 levels have never been above 300 ppm since humans have been on earth.

Atmospheric Carbon Dioxide over past millennia.
Atmospheric Carbon dioxide over past millennia (NASA)

Over the earth’s 4.5 billion year history, it has mostly been hotter than it is now, and generally hotter than would allow modern life. It has occasionally been colder too.

Estimated global temperature over 500 million years.
Estimated global temperature over 500 million years (NOAA)

But most of this was long before humans existed on earth. Since humans have been on earth, temperatures have never been as high as they are now, nor have they ever risen this quickly.

Global temperature over past 20,000 years.
Global temperature over past 20,000 years (Volewica). I am seeking permission to use this.


This means that we have started to move outside the “safe climate zone”. Experts suggest that, if nothing changes and we stay with current climate policies, by the year 2040 the earth will be 2 degrees centigrade above the safe zone.

This means there is more energy trapped in the earth and it’s atmosphere. And more energy means more instability, more extremes of weather, more storms, more droughts, more bushfires.

For more detail on the consequences, see How global warming leads to climate change (coming soon).


We use a lot of NASA and NOAA material because it is freely available.

Main graphic: 2013 wildfire in California (Wikipedia)