Causes of global warming

The causes of global warming are about simple physics.

To warm the thin skin of the planet — the tiny part that life inhabits — there must be an increase in the net energy in the atmosphere and oceans. The balance of energy gained and lost must shift slightly.

For warming to happen there must be

• an increase in the energy received into the atmosphere and oceans
• an increase in the energy retained by the atmosphere and oceans
• or both.

As the sun is the source of energy for the atmosphere, warming is only possible if the incident energy from the sun either becomes more intense and/or more of the suns energy is retained in the atmosphere and oceans.

Energy received

The energy received by the sun does vary in its intensity — the technical term is solar luminosity.

This happens because

1. the earth is closer to the sun at various times in its orbit, notably when the earth's orbit wobbles, and
2. because the sun emits energy at varying intensities.

Both of these causes are significant.

Energy retained

The energy retained in the atmosphere is the proportion of energy received that is not reflected back into space or absorbed by the earth and oceans.

This is itself related to two things

1. the composition of the atmosphere and
2. the reflectance (albedo) of the land

Some of the energy from the sun that bounces back towards space as longer wavelengths is trapped by gases in the atmosphere that let the shorter wave incident radiation pass through but hold on to the longer wavelengths on their way out.

We have known for a long time that water vapour, carbon dioxide, methane and nitrous oxide, together with several man-made gases such as CFCs, have these energy absorption properties and in the atmosphere they create a greenhouse effect (also called radiative forcing). Indeed without them the earth would be a ball of ice.

We also know that when the proportion of greenhouse gases increases, so does the amount of reflected energy retained in the atmosphere.

The albedo (reflectance) of the land surface also has an effect on the amount of energy that is reflected back into the atmosphere. Generally vegetation absorbs more energy than agricultural land and urban areas.

Particles in the atmosphere (aerosols and soot) are also important, especially when dust and ash increase as a result of major volcanic eruptions. 

The political, media and public debate of climate change exists because human activity has altered the albedo of the earth, the level of particulates in the atmosphere and the proportion of greenhouses gases (CO2, CH4, N2O and CFCs).

So the simple explanation for the causes of global warming becomes an increase in the proportion of greenhouse gases.

And because we see most of the increase in these gases as a result of human activities — especially land clearing and burning of fossil fuels — we suspect warming is our fault. We even give it a name, anthropogenic global warming

the conundrum

It is easy to think that because we engineer everything, there are now so many of us, and for several generations we have cleared land and burnt fossil fuels, it is us, human beings, that are responsible for global warming.

Indeed, it is unlikely that we are not having some kind of effect on the global and local climate. 

But we should not forget that there are still major forces at play too, other causes of global warming that are much stronger than us. We cannot stop our planet wobbling in its orbit around the sun nor can we alter the amount of energy the sun releases.

If ocean currents shift, then they will shift and affect climate. They are forces that are beyond our control.

We can control the amount of greenhouses gases our activities release and the extent of albedo change that is a result of our use of the landscape. Whether these levers are enough to control the amount the climate changes is unknown.

So far we have chosen not to pull them too hard. We still burn fossil fuel and clean land to meet immediate needs.

We can also be sure that whatever the global warming effects are,  they will be more acute on us, on our economic systems and wellbeing, than changes in the past — simply because there are now so many of us.

This will leave us with only one way out, to adapt.

Ultimately the causes of global warming is a net increase in the energy held in the atmosphere and oceans — more energy received and/or retained than before. That this is in part to do with the concentration of greenhouse gases is beyond dispute. But is is not the only cause and so will not be the only route to a solution.