THE CAUSES OF CLIMATE CHANGE

THE CAUSES OF CLIMATE CHANGE

We’ve established that global warming is happening and this is leading to climate change, now we’ll look at what might be causing it and whether it’s natural, manmade or both.

THE GREENHOUSE EFFECT

Our atmosphere acts like a blanket trapping heat and keeping Earth at a habitable temperature, it’s this retaining of heat that is referred to as the Greenhouse Effect. The greenhouse effect is caused by greenhouse gases that trap heat from the sun, the more greenhouse gases there are the more heat is retained (Wikipedia - Greenhouse Gas).

GREENHOUSE GASES

Water vapour (H20) is the most prevalent of all the greenhouse gases and depending how humid the air is, it can account for up to 4% of the composition of air (note 1). Excluding water vapour, the air comprises 78.084% nitrogen (N2), 20.946% oxygen (O2) and 0.9340% argon (Ar); together these account for 99.9% of the composition of air. However, they do not have the same heat retaining capacity of other gases and are not considered greenhouse gases (Wikipedia - Greenhouse Gas).

Water vapour can be considered to be a natural greenhouse gas (note 2). Some greenhouse gases are both natural and manmade including carbon dioxide (C02), methane (CH4), nitrous oxide (NO) and tetraflouromethane (CF4). There are several synthetic gases consisting of carbon and halogens (note 3), the group of gases called chlorofluorocarbons (CFC’s) are perhaps the best known of these. Many of the manmade greenhouse gases are also responsible for ozone depletion (EPA - Ozone).

As with temperature, there is a natural cycle in the amount of CO2 in the atmosphere. Ice core samples extending back some 650,000 years show the minimum amount of atmospheric CO2 to have been around 190 ppmv (note 4) and the maximum about 300 ppmv [see graphs below]. The worry is that the current levels of CO2 are considerably higher at around 385 ppmv [San Francisco Chronicle].

LEVELS OF GREENHOUSE GAS

From the table below we can see that, excluding water vapour, carbon dioxide is by far the most prevalent of the greenhouse gases accounting for over 99% of all of them. The only other gas present in appreciable quantity is methane and this accounts for nearly 0.5% of the greenhouse gases. Between them, these two gases account for more than 99.9% of all greenhouse gases.

By comparing levels of greenhouse gases in 2000 with levels in 1750 we can see that there has been a 31% increase in the amount of carbon dioxide, a 16% rise in levels of nitrous oxide and a 149% increase in the levels of methane. Several of the gases are entirely synthetic and didn’t exist back in 1750.

Different greenhouse gases are more effective than others at contributing to the greenhouse effect, an effect called the global warming Potential (GWP). Although carbon dioxide accounts for 99.4% of the greenhouse gases by volume, the contribution it makes as a total of all the greenhouse gases is considerably lower at just 72.3%; this is because, as greenhouse gases go, it’s not very good. Nitrous oxide on the other hand is nearly 300 times as effective and although it occurs in very small amounts when compared to carbon dioxide, it manages to contribute 18.4% towards the total greenhouse effect. The other gases which make sizeable contributions are methane and dichlorodifluoromethane, respectively these are responsible for nearly 8% and nearly 1% of the contribution to the greenhouse effect.

CHANGES IN LEVELS OF GREENHOUSE GAS

Four gases are responsible for 99.5% of the greenhouse effect - carbon dioxide (72.3%), nitrous oxide (18.4%), methane (7.9%) and dichlorodifluoromethane (0.9%). Below are three graphs showing how the levels of three of these gases have changed during the last 1000 years, there is no graph for dichlorodifluoromethane (freon) as this is a man-made gas and didn’t exist until quite recently and it’s production has now been discontinued.

Carbon Dioxide

Levels of carbon dioxide in the atmosphere have been fairly constant at around 280 part per million, in the last 200 years there has been a marked rise in levels. In the 250 years between 1750 and 2000 levels increased by 31.3%. Current levels are 385ppmv.

Nitrous Oxide

Historically levels of nitrous oxide have been between 260 and 275 parts per billion. In 1750 there were 270 ppb, by 2000 there were 314 ppb, a rise of 16.3%.

Methane

In 1750 the levels of methane in the atmosphere were 700 parts per billion, about the same as they had been previously. By 2000 the levels had risen sharply to 1745 parts, a rise of 149.3%.

SOURCES OF GREENHOUSE GASES

One of the primary causes of greenhouse gas emissions for which humans are responsible result from the burning of fossil fuels (coal, oil and gas) and much of the remainder results from farming and agricultural activities. Fossil fuels are used in power generation, to heat homes and offices, to power factories, to fuel transport and many more uses.

Other manmade causes of greenhouse gases include deforestation (note 6), fertilisers, air conditioning units, open fires, fridges and freezers, numerous industrial and chemical processes, fire suppressants, coal mining, effluent, landfill sites, livestock and rice cultivation [Knauer Group].

There are several natural causes of greenhouse gases including volcanic activity, the seas and oceans, natural decay of plants and animals and the natural melting of ice caps.

THE CARBON CYCLE

There’s a natural cycle of carbon dioxide. Annually 120 billion tons of CO2 are absorbed from the atmosphere by vegetation and soil whilst at the same time 119 billion tons are released into the atmosphere. A similar exchange takes place between the oceans and the atmosphere with 90 billion tons of CO2 being absorbed and 88 billion tons being released. The net effect is that each year the atmosphere loses 3 billion tons of CO2.

One very important factor not taken into account here is the anthropogenic effect. In 2006 humans added 7 billion tons of CO2 to the atmosphere just through the burning of fossil fuels resulting an overall increase in atmospheric CO2 of 4 billion tons.

Carbon dioxide is exchanged between the atmosphere and the oceans and between the atmosphere and vegetation, this is the natural cycle and results in 3 billion tons of carbon dioxide being lost from the atmosphere each year. In 2006 humans added 29 billion tons of carbon dioxide to the atmosphere resulting in an overall increase of 26 billion tons.

CARBON IMBALANCE

During the last 400,000 years levels of carbon dioxide have fluctuated between 190 and 310 ppmv. In the last 200 years levels have risen sharply to 385 ppmv.

Enlarge - More info and explanation Left to it’s own devices the natural carbon cycle is more or less in equilibrium and significant changes to levels of carbon dioxide in the atmosphere occur over timeframes spanning many thousands of years and correlate with the ice age cycles.

The first graph to the left show how concentrations of CO2 in the atmosphere have changed over the last 400,000 years with levels fluctuating between 190 and 310ppmv (note 7). The inset graph shows that for the last 1000 years atmospheric CO2 levels have been consistent until approximately 250 years ago when they began rising rapidly.

The peaks and troughs are quite consistent, lows of around 190ppmv and highs of around 300ppmv. Coming out of a recent ice age we would expect to have seen levels peak at around 300ppmv and then start to fall again. Indeed, up to 250 years ago levels were behaving as would have been expected but then began rising dramatically. The third graph shows that in the last 50 years levels have risen from 315ppmv to 380ppmv, a rise of 65ppmv. Compared to natural variations this is at least 100 times as fast as would be expected, even when natural levels are increasing at their fastest.

The graph in the middle illustrates that over periods of hundreds of millions of years there are significant changes in the levels of atmospheric carbon dioxide with some sources estimating that levels half a billion years ago were as much as 18 times as high as they are now. Estimates this far back rely on oxygen isotope analysis and only give an indication of the approximate levels of gases that may have been present. A much more accurate way of analysing the atmosphere from years gone by is by extracting air trapped in ice, using this approach there is now an accurate record dating back 650,000 years.

250 years ago the Industrial Revolution began (note 8), the demand for energy started rising and has continued rising ever since. Not only are there far more people living on the planet than ever before but the average amount of energy each person uses grows year on year. Today over 80% of the world’s energy still comes from fossil fuels, before the days of nuclear power and renewable energy sources it was even higher.

Industrialisation is by far the biggest factor in the carbon imbalance but there are others including deforestation. Carbon dioxide plays a vital role in nature in that it enables plants to photosynthesise (note 9) and each year some 17 billion tons of CO2 are absorbed in this way, this is just one of the natural processes which in total absorb 120 billion tons of CO2 a year. For many decades huge swathes of forest have been cut down and although new forests are being planted the net loss is still about 100,000 square kilometres of forest a year [UN Report]. With an ever decreasing amount of forest there is a corresponding decline in the amount of carbon dioxide absorbed for photosynthesis.

SOLAR VARIATION

Solar variations cause changes in the amount of heat energy emitted from the sun and there are many reasons for this. The primary variation is caused by solar cycles, a pattern that repeats itself at 11 year intervals. However, the difference between the highest and lowest output is very small - about 0.1% [Wikipedia - Solar Variation]. There are many other cycles that the sun goes through ranging from 22 years to several thousand years; there may be longer ones that haven’t yet been detected.

The heat energy received from the sun is 1366 W/m²/yr (Watts per square metre per year) and it’s estimated that since 1750 this has increased by 0.3 W/m²/yr. During the same time, anthropogenic global warming has contributed a further 2.4 W/m²/yr [IPCC].

In the short term at least, solar variation has little effect on global warming and climate change. Over many thousands and millions of years the changes are significant and together with other astronomical factors (note 10) can explain globalwarming awareness2007 and cooling cycles over long periods of time.

FEEDBACK OR COMPOUND PROCESS

Globalwarming is a self-perpetuating cycle. If we take the frozen tundra of Siberia as an example, the permafrost here is melting and in the last few years one million square kilometres has melted [The Guardian]. Trapped in the peat beneath the permafrost is up to 70 billion tons of methane, as the ice melts the methane is released into the atmosphere. Methane, as we’ve established, is a more effective greenhouse gas than carbon dioxide and releasing this much methane has the same globalwarming awareness2007 potential as releasing 1.6 trillion tons of CO2.

The release of the methane increases global warming which leads to more ice melting, more methane released, more ice melting...

Further, as the ice melts it exposes the land beneath and forms melt-water lakes, both land and water are less reflective than ice so they absorb more solar heat radiation, further adding to Global warming and the feedback process.

Global Warming causes an increase in the levels of greenhouse gases. Greenhouse gases cause an increase in the levels of global warming.