Climate Change Last years' Nobel Peace Prize was awarded to Al Gore and the Intergovernmental Panel for Climate Change (IPCC) for increasing awareness about climate change. We read an article on this in the last issue of JM. Why has climate change become an issue? This is because all evidence leads us to conclude that the average temperature of the Earth's near-surface air and oceans has been steadily increasing in recent decades. This observation has been named global warming and it is by now clear that human beings have greatly contributed to it. <02.01.jpg> The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C during the last 100 years. The IPCC states that most of this is due to increase in greenhouse gas emissions from human-related activity. This finding has been confirmed by several independent analysis. <05.24.jpg> Effects of global warming What does this mean? Increase in industrialisation over the whole world indicates that global warming will only increase. Most projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C during the 21st century. This will lead to a rise in sea levels as well as melting (retreat) of glaciers in the poles and in high mountains such as the Himalayas. The retreat of ice of most glaciers in the Himalayas is already well-documented. The cover picture shows the retreating edge of the Gangotri glacier which is the source of the river Ganga. While is is known to have retreated for more than 200 years, the rate of recession has increased after 1971. Both temperature and sea level rise are expected to continue for 1000s of years even if we stabilise the levels of greenhouse gases today. This is because a large amount of heat is trapped in the oceans, since water has a very large heat capacity (lots of heat is required to increase the temperature of water by only a small amount). The rise in sea-level will submerge a lot of cities that are on the sea-coast, resulting in displacement of large populations. Also, increasing temperatures and change in salinity of the oceans (salt content due to large addition of fresh water from melting ice) will cause a change in weather patterns, with more frequent occurrence of extreme weather events such as cyclones. Is this a doomsday warning? No. It is rather a red alert. The time to act is now. The IPCC sets out clearly what action should be taken to reduce or reverse future warming. Indeed, most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions. What remains to be seen is how well the proposals will be converted to definite action. Box Greenhouse gases Some gases in the atmosphere emit long wavelength radiation. This is more readily absorbed by the atmosphere than, say, sunlight. The part that is emitted away from the Earth is lost to space. The part emitted in the direction of the Earth remains trapped in the atmosphere and warms it. This is called the greenhouse effect (which has nothing to do with the principle of greenhouses). Naturally occurring greenhouse gases have a mean warming effect of about 33 °C, without which Earth would be uninhabitable. On Earth, the major greenhouse gases are water vapour which causes about 36-70% of the greenhouse effect (not including clouds); carbon dioxide which causes 9-26%; methane which causes 4-9%; and ozone which causes 3-7%. The amount of of carbon dioxide and methane in the atmosphere have increased by 31% and 149% respectively above pre-industrial levels since 1750. While this may not seem a lot, these levels are much higher than at any time during the last 650,000 years! Greenhouse gas emissions and human activity The major source of greenhouse gases due to human activity is from burning of fossil fuels (petrol, coal, natural gas, etc.), and deforestation. Methane gases are also generated by live-stock like cattle when they belch, and even from paddy/rice farming! Paddy fields are submerged with water so that oxygen supply to the soil gets cut off. This results in anaerobic fermentation of organic soil matter, whose major end-product is methane (C H_4). Agricultural activities (such as the use of fertiliser) also leads to increased nitrous oxide concentrations. <350px-Greenhouse_Gas_by_Sector.png> Another source is the use of chloro-fluoro-carbons (CFCs) in refrigerators, although refrigerators are now being manufactured without these. That is because the use of CFCs was restricted because they cause depletion of ozone in the atmosphere. Water vapour is a naturally occurring green-house gas and accounts for the largest percentage of the green-house effect, from 36-66%. It is believed that human activity does not directly affect water vapour concentrations globally. However, increase in atmospheric temperature will lead to an increase in the water vapour content of the atmosphere, which will in turn increase the green-house effect, leading to a further increase in temperature. This cycle is called a positive feed-back cycle, since one condition worsens the other. Measurements from Antarctic ice cores show that atmospheric CO2 levels were about 260-280 parts per million (ppm) by volume (1 million cc of atmosphere will contain 260-280 cc of CO2) during the last 10,000 years! Since the beginning of the industrial revolution, the concentrations of many green-house gases have increased: CO2 is now about 380 ppm, which is an increase of 31%. Methane and nitrous oxide have increased by 150% and 16% respectively. From less direct geological evidence it is believed that carbon dioxide values this high were last attained 20 million years ago. The main contributors are fossil fuel (petrol etc) burning and deforestation. How long do these gases stay in the atmosphere? Water vapour stays only a few days in the atmosphere before it is transported away as rain or snow. Other green-house gases stay in the atmosphere for a long time. Methane is oxidised (changed to water and carbon-dioxide) in the atmosphere. Carbon-dioxide is removed by plants during photosynthesis which is their energy source. It is also dissolved in the oceans. So methane has a life time of only about 12 to 15 years. This is the amount of time takwn for a small excess to be removed from the atmosphere. Nitrous oxide has a lifetime of 120 years, while CO2 has an atmospheric lifetime that can be tens of thousands of years. So, even if you stop all green-house emissions today, there will still continue to be global warming for many more years. It is therefore even more crucial to limit gree-house gas emissions. So, how do you stop this run-away increase in green-house gases? You must find ways to decrease emissions, or find sources that will absorb the gases. For instance, it is known that forests are important stores of carbon and can extract carbon dioxide from the air during photosynthesis. Humans can curb or curtail their carbon-intensive activities, by saving on fuel and energy use. BOX Sea ice versus land ice When experts talk about rise in sea levels, this does not come from all the ice that is melting. There is a lot of ice that is floating on the sea. One such famous iceberg is that which sank the ship RMS Titanic in 1912, when about 1500 people died. This happened because when ice floats, a large part of it remains concealed under water. Only about 15% is visible, so it is difficult to estimate its extent. What does this have to do with global warming? Well, when such floating ice melts due to an increase in temperature, there is hardly any impact. It is when ice that has remained on land (such as from glaciers) melts and the water flows into the sea that the sea-level rises. So the ice of most concern to scientists is the land-based ice in mountains and in Greenland and Antarctica, which are very large bodies. The Greenland ice sheet has been observed to be shrinking since 1978. Scientists believe that Greenland's massive ice sheet could begin to melt and may disappear within the next thousand years if global warming is not checked. If this happens, the sea levels may rise by as much as seven metres, which would be catastrophic for many coastal cities and nations. Adapted from various articles, including Wikipedia, http://en.wikipedia.org