Why is the climate changing?
UNESCO, Nairobi, Kenya
Greenhouse Gases, the Greenhouse Effect and Global Warming
The Earth receives energy from the sun in the form of ultraviolet rays (light) and releases some of this energy back into space as infrared rays (heat). Gases can absorb some of this outbound energy and re-emit it as heat. These gases – which include water vapour, carbon dioxide, methane, nitrous oxide and others – are called ‘greenhouse’ gases. They act like a blanket that surrounds the Earth and keeps it warmer than it would otherwise be, just as the glass panes of a greenhouse allow the sun’s energy to enter but prevent some of the heat from escaping. Without this natural process, known as the greenhouse effect, our planet would be on average about 30 degrees Celsius cooler, so the greenhouse effect is essential. But too much of an effect will create problems. Human activities over recent generations have artificially raised the concentration of greenhouse gases in the atmosphere and scientists conclude that this is why the planet has warmed in recent history. But, because greenhouse gases can last in the atmosphere for a long time, even if all emissions worldwide stopped today, the climate would continue to change.
The greenhouse effect is not a new discovery. Joseph Fourier discovered it in 1824, John Tyndall experimented on it in 1858, and Svante Arrhenius quantified it in 1896. Since then scientists have provided growing evidence not only that the concentration of greenhouse gases in the atmosphere has increased, but also that this increase threatens to cause dangerous climate change. Measurements from Antarctic ice cores show that for about 10,000 years before the Industrial Revolution the concentration of carbon dioxide in the atmosphere was about 280 parts per million (ppm) by volume. Since then it has risen rapidly: in 2013, the concentration reached 400 ppm, a threshold that last occurred over three million years ago. Then, the world on average was 3-4 degrees Celsius warmer than it is today and sea levels were much higher.
What emits greenhouse gases? Whose emissions are they?
Major sources of greenhouse gas emissions from human activities include power generation (about 25 per cent of all emissions), transport, industrial activities, deforestation and agriculture. Countries have historically varied greatly (and continue to today) in the type, source and amount of greenhouse gases they emit. The biggest emitter overall today is China, but its large population means that emissions per person (per capita) are lower than in many other countries. Historically the United States has emitted more greenhouse gases than any other country, and today its per capita emissions are still among the highest worldwide: 100-200 times greater than per capita emissions in most African nations. The question of who is responsible for climate change becomes complicated when consumer demand in one country increases emissions in another.
Africa’s emissions are low in both absolute and per capita terms. Total emissions for Africa increased twelve-fold between 1950 and 2008, reaching 311 million metric tons of carbon, which is still less than the emissions for some single nations including China, the United States, India, Russia, and Japan. Although per capita emissions in 2008, at 0.32 metric tons of carbon, were three times those in Africa for 1950, they were still only 6.6 per cent of those in North America.
Emissions from all fuel sources have grown in the African region over time with liquid and solid fuels now each accounting for approximately 35 per cent and gas fuels accounting for 17 per cent of the regional total. A small number of nations are largely responsible for the African emissions from fossil fuels and cement production; South Africa accounts for 38 per cent of the continental total, and another 46 per cent comes from Egypt, Algeria, Nigeria, Libya and Morocco combined.
These are the only six countries on the continent with annual CO2 emissions in excess of 10 million metric tons of carbon. Only four African countries have per capita CO2 emissions higher than the global average (1.3 metric ton of carbon per year): Libya (2.53), South Africa (2.39), the Seychelles (2.22), and Equatorial Guinea (1.99). Based on 2008 per capita emission rates, 28 of the 55 African nations for which data are available have per capita emission rates less than 0.1 metric ton of carbon per person per year.
What else affects the global climate?
Greenhouse gases are not the only things to affect the temperature of the atmosphere and the Earth. The sun’s rays vary in strength. Periodic events called El Niño and La Niña alter the circulation of warmer and cooler water in ocean currents, leading to changes in climatic patterns across large regions. Clouds reflect sunlight back into space and, in doing so, reduce the amount of energy that reaches the Earth. And when volcanoes erupt they produce tiny particles that also reflect light energy in this way. Conversely, particles of black carbon or soot absorb heat. Transport fuels and burning forests and vegetation produce these particles, which scientists think have a warming effect about two-thirds as strong as that of carbon dioxide.
How much heat?
Global warming is just that – global. It refers to the worldwide average increase in temperature above a long-term average. The global average temperature rose by about 0.85 degrees between 1880 and 2012, and the rate of warming has accelerated over the past 50 years, according to the Intergovernmental Panel on Climate Change. Global averages mask big differences in warming between regions. In general there is more warming over land than the oceans and more warming at the poles than in the tropics. Global warming does not increase at a constant rate. From year to year the global average temperature can increase or decrease, but over decades the warming trend is clear. The reasons for these variations include the fact that much of the excess heat that greenhouse gases trap moves into the oceans, including into deep waters.
How much more heat?
Climate sensitivity is the term scientists use to explain how much the temperature will change because of the factors that affect the climate system. One of the main ways to understand climate sensitivity is to ask how much the temperature will rise if the concentration of the greenhouse gas carbon dioxide doubles to 560 parts per million (ppm) from 280 ppm, the level it was before the Industrial Revolution. On current trends, this will happen between 2050 and 2070. Scientists differ in their estimates of how much the world will warm as a result. The Intergovernmental Panel on Climate Change’s Fifth Assessment Report said in 2013 that this figure was “likely to be in the range 2°C to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C”.
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