AP
This 2006 file photo shows a polar bear watching a whaling crew offshore in Alaska.
In two weeks, thousands of people from around the world will converge on Durban for the start of the annual climate change negotiations.
The UN negotiations are political, with nations trying to decide who is going to do what about climate change and when. After years, they have not reached consensus. However, what delegates do agree on is the climate is changing. Here is some of the evidence.
How do we know the climate is changing?
From scientific measurements over time. Climate scientists first sounded the alarm in the 1960s and 1970s when they noticed a steady increase in global temperatures. Many different agencies around the world record thousands of land and ocean temperatures, and use different techniques to process the data. All these independent methods show a warming trend. This is confirmed by other phenomena: melting of mountain glaciers, reduced sea ice in the Arctic and rising sea levels.
Why is the global climate getting warmer?
Mainly because human activity has changed the natural “greenhouse effect”. The greenhouse effect is essential for all life on Earth. Greenhouse gases, such as CO2 and methane, occur naturally in the atmosphere. We could not live without them because they keep some of the sun’s warmth from escaping back into space. Without greenhouse gases we would freeze. The problem is that human activities are changing the “natural greenhouse”. Since the Industrial Revolution, we have burnt fossil fuels at an increasing rate, pumping higher levels of CO2 into the atmosphere. To a lesser extent, cutting down forests and some forms of agriculture have added to the greenhouse gases. It is a matter of scale: some greenhouse gases are essential for life. Too much, and we slowly start to heat up.
Have there have been changes in the levels of CO2 in the past?
Yes, CO2 levels have gone up and down over the past 650 000 years. However, in all this time the CO2 level has never been above 300 parts per million – until 1950. The current CO2 level is 389 parts per million.
Isn’t the sun to blame for the warming?
No. While changes in the amount of energy the sun gives off played a part in past climate changes, evidence shows that current global warming cannot be explained by changes in the sun’s energy. In fact, satellite data since 1978 shows a slight drop in the amount of energy from the sun.
What is happening to the ocean?
There is a strong trend of warming of the ocean’s upper levels. As the ocean heats up, the water expands and sea levels rise. Melting glaciers also contribute to sea level rise. Global sea levels have risen about 17cm in the past century. The rate in the past decade is nearly double that of the past century. As the ocean absorbs more carbon, it becomes more acidic. This affects marine life.
There have been climate changes the Earth’s history. What is different now?
The current warming trend is different because it is very likely that most of it is happening because of human activity, particularly burning fossil fuels. A large body of scientific evidence supports the conclusion that human activity is the main driver of global warming. Also, it is happening faster: Earth has warmed twice as fast in the past 50 years than in the 50 years before that. The average temperature of the Earth’s surface has risen by 0.74°C since the late 1800s. Most of the warming has occurred since the 1970s. The 10 warmest years on record have occurred in the past 12 years.
Why is there such a fuss about warming of less than one degree?
Small changes in the global average correspond to enormous changes in the environment. For example, in the last Ice Age, when much of Asia, Europe and North America were covered by ice, the average global temperature was only about five degrees cooler than today. At today’s rate of CO2 emissions, with the current pledges of nations to cut emissions, we are set to see the average increase by about 3.5°C this century, creating a world no humans have experienced.
l Sources: Nasa; National Climatic Data Centre; UN Framework Convention on Climate Change.
