What is a Greenhouse Gas?

• A greenhouse gas is part of the atmosphere that can capture and re-emit infrared (heat) that is being radiated towards space from a warm Earth. As sunlight adds more heat to the Earth, the greenhouse gases capture and re-emit the heat back, so that the overall heat is increased. The greenhouse gases trap the heat so it cannot escape to space. Because the heat is trapped, it causes the temperature of the atmosphere, land, and ocean to rise.
• Greenhouse gas molecules all have at least three atoms, a property that allows them to vibrate relative to the central atom. This is how they they capture infrared radiation and re-emit it back toward the Earth. The predominant greenhouse gases in our atmosphere are water vapour, CO₂, methane, nitrous oxide and fluorinated gases (CFCs).

How do we know the CO2 is from fossil fuels and not volcanoes?

• The first part of the answer is that people are adding a great deal more CO₂ to the atmosphere than is coming from volcanoes.
• The second part of the answer is based on the fact that plants prefer to use the lightest isotopes of carbon so choose C¹² over C¹³. Volcanoes produce CO₂ with both C¹² and C¹³. Fossil fuels are made from ancient plants and animals. If the carbon is being added to the atmosphere from fossil fuels, then it should have much more C¹² than C¹³. Over time we should see the proportion of C¹² increase in the atmosphere and that is exactlly what is happening. Because we are using so much fossil fuel, we are overwhelming the volcano-produced CO₂ so the proportion of C¹³ to C¹² has been going down since the start of the industrial age.
• The third part of the answer is to distinguish between modern and ancient carbon in the CO₂. Carbon from fossil fuels is ancient. C¹⁴ is normally present in the air so organisms also store C¹⁴ in their bodies. C¹⁴ has a special characteristic. This isotope of carbon has a half-life of less than 6,000 years. That means that fossil fuels, which are millions of years old, will not have C¹⁴. If the isotope C¹⁴ is absent from fossil fuel CO₂ but C¹⁴ is present in the atmosphere normally, then adding fossil fuel carbon should reduce the proportion of C¹⁴ in the atmosphere over time. The answer to this part is yes, C¹⁴ has been declining steadily.

When did Global Warming begin?

• Natural global warming and cooling has been going on for a long time - millions of years. But human caused global warming is caused by burning fossil fuels and poor land use and forestry practices. It is only in the last 250 years that people have known how to burn coal, oil, and natural gas in large amounts to use in industry. So the first real start of anthropogenic global warming was in the middle 1700s.

What is the difference between global warming and climate change?

• Global warming is specifically the increase in temperature of the atmosphere, soil, plants and ocean.
• Climate change is more general and refers to any change in the long term expectation of weather conditions. The term was popularized by Frank Luntz - PR expert - as a strategy for President Bush of the USA to reduce the level of fear and anxiety that the term global warming created.

How can puny humans affect the entire world?

• We may be puny, and it may be that one person cannot control the climate, but more than 7,000,000,000 of us can do a lot of work and produce way more CO₂ than volcanoes have been producing.

Why are clouds a wild card in climate models?

• Skeptics often remark that models consistently ignore the magnitude of the effect of clouds – not just today, but into the future, because we do not have the capacity yet to predict if the cloud cover will increase or decrease under future conditions. The basic question from the perspective of what do we do about this is: “How much difference does it make?” Turns out it could be quite a lot (and that is reflected in the large uncertainty ranges in climate models) but it is also likely not to make a real difference in the most likely scenario.
• As a general observation (measured at the top of the atmosphere) clouds act both as positive and negative influences. On a clear cloudless day the Earth will absorb about 20% more heat from the sun than the current Earth does. To achieve a radiant balance with no clouds the Earth would need to be about 12⁰C warmer. Clouds have two effects. They reflect sunlight back to space before it can heat the Earth and they also have a heating effect by inhibiting the radiative loss to space. This warming effect is about 7⁰C. The net effect of clouds therefore is about a 5⁰C cooling. What we do not know with any certainty is if clouds will increase or decrease on average in the future. If the average cloud cover increases, the net effect would be to add a cooling factor to the global warming. By contrast if the cloud cover decreases, the net effect will be to enhance the global warming.
• This next is a bit crude, but here is the idea. If the clouds increase by just enough that the net increase causes a further cooling effect to a little less than the increase in heating due to CO₂, the net effect will be a minor increase in overall global temperature increase – the rate at which we are currently warming. To suggest we “know” what the future holds would be incorrect.
• On the other hand, the basic frequency of clouds is largely, but certainly not entirely determined by the planet’s surface characteristics – more open water produces more clouds, dry soil in the desert creates few clouds. Currently some 70% of the planet on average is covered by clouds. Warming trends could promote more clouds. But there are essentially two types of clouds relevant to this question: cooling clouds (low stratus clouds reflect a lot of sunlight and are cooling clouds, cirrus clouds both reflect and trap heat for a net warming effect, and cumulus clouds do one or the other depending on how thick and abundantly distributed they are).
• The end of all this research is not in sight yet, but in terms of net effect, the cloud changes might slow down or increase the rate of global warming depending on whether the average cover increases or decreases and also depending on what types of clouds might increase or decrease. This explains the degree of uncertainty shown in the models (because the clouds are just established as parameters and the uncertainty is whether the cloud distribution in the future being very different from the present) but does not change the most probable trends.

How much of global warming is due to human causes?

• More than 100%! How do we know this?
• The climate should be cooling according to the natural causes of climate change. The climate is not cooling because people have been adding greenhouse gases to the atmosphere. The net effect is that people are responsible not only for the warming but also for overcoming the cooling – so the contribution of people is actually more than 100%.

I thought water vapour was the only important greenhouse gas. Everything else is just a trace gas?

• Water vapour is indeed the major greenhouse gas in the atmosphere. Water vapour provides most of the moderating effect on our climate without which we would have an unliveable climate.
• However, it is largely in equilibrium (absolute humidity) because the amount of water vapour in the air mostly (but not entirely) depends on whether it is over land or water, and whether the land is desert or moist. The total amount of water the air can hold is dependent on the temperature (relative humidity). If the relative humidity reaches 100% at any given temperature and the temperature falls, the water comes out of the air as rain, snow, or ice because the relative humidity cannot rise above 100% and a falling temperature reduces the amount of water the air can hold.
• Thus, while water vapour is the major stabilizing influence in the climate, it can only act as a feedback mechanism after some other factor raises the temperature allowing more water vapour which in turn enables a greater greenhouse gas effect.