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How are people changing the climate? Read more Unit 1: 1. Man-made climate change? - more In the Basics-section you will find links to the Read more-section. Follow those links if you want to explore an issue in more depth (rather than reading this Read more-section from beginning to end). In this unit you can read about: - Observed changes in concentrations of greenhouse gases in the atmosphere Observed changes in the Earth's climate The inertia of the climate system Feedback effects Abrupt changes. This unit also has worksheets. Part 1: Changes in concentrations 1.Man-made climate change? Observed changes in concentrations of greenhouse gases in the atmosphere. Carbon dioxide (CO2) has increased from 280 ppm (280 litres of CO2 in a million litres of air) in the period 1000 to 1750 to about 370 ppm today. This is about a 31% increase and today’s levels are the highest atmospheric CO2 concentrations in at least 420,000 years, probably 20 million years. 1. Increase in greenhouse gas concentrations (in ppm) from 1750 to 2000. Graphics: Elmar Uherek. Methane (CH4) has increased from 0.7 ppm in 1750 to 1.7 ppm in 2000. This is an increase of about 151%. Nitrous oxide (N2O) has increased from 0.27 ppm in 1750 to 0.32 ppm in 2000. This is about a 17% increase. Tropospheric ozone (O3) has increased by about 35% from 1750 to 2000. This figure, does however, vary from region to region. Ozone in the lower atmosphere is a greenhouse gas, it is formed and broken down by chemical reactions in the atmosphere, and humans emit other substances which influence these chemical reactions. (Source: IPCC 2001 and http://cdiac.esd.ornl.gov/pns/current_ghg.html). ESPERE Climate Encyclopaedia – www.espere.net - People changing the Climate More - page 1 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics Part 2: Changes in climate 1. Man-made climate change? Observed changes in the Earth’s climate Observed changes in the Earth’s climate The 1990's appear to be the warmest decade since 1860 and 1998 was the the warmest year. Over the last 1000 years, the 1900's probably saw the greatest temperature increase in the Northern Hemisphere. Less is known about the Southern Hemisphere. Weather Average global temperatures on both the land and the ocean surface have increased by 0.6°C. Temperatures have increased more over land areas than over the oceans. The differences between day and night temperatures have decreased because night temperatures have increased more than day temperatures. The number of hot days has increased. The number of cold days with frost has decreased. The average rainfall in the Northern Hemisphere increased by 5–10% throughout the 1900's. Some areas, such as the Mediterranean region and the northern and western regions of Africa, have had less rain, while other areas have become wetter. The number of extreme rainfall events at mid- and high-latitudes (high latitudes are regions close to the North or South poles) have increased. Areas in Asia and Africa have had more frequent and more intense dry periods during the summer. Other physical conditions Sea level has increased, on average, by 1 - 2 mm per year during the 1900's. The period when ice covers lakes and rivers became about two weeks shorter per year during the 1900's. Glaciers outside the polar regions have receded. 1. Traces of the past: By drilling deep into mountain glaciers and the ice caps on Antarctica and Greenland, scientists can analyze old ice to find out about the climate in the past. The ice core on the picture contains dust from a volcanic eruption, carried by the wind from far away. Photo: Marzena Kaczmarska/NPI Permafrost (ground or bedrock that is always frozen) in polar and mountain regions has thawed. El Niño is a weather phenomenon in the Pacific Ocean that appears regularly, ESPERE Climate Encyclopaedia – www.espere.net - People changing the Climate More - page 2 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics about every five years around Christmas (which is why it is called “El Niño”, as this means “Baby Jesus”). El Niño has always affected the climate in large areas of the world and led to drought and catastrophic flooding. Scientists believe that global warming may make El Niño more intense and occur more often. This trend has been visible over the last 20 to 30 years. Part 3: Inertia of climate The inertia of the climate system The inertia of the climate system One important characteristic of the climate system is it’s inertia – that is, it takes a long time from when something happens (cause) to when we see the total consequence (effect). For example, it takes a very long time from: when people emit CO2 or another gas until we can measure new stable concentrations of the gas in the atmosphere. when the concentration of greenhouse gases increases until we can see the effect on the temperature. when the temperature changes until we can see the biological effects in various species of plants and animals such as their extinction, mutations, or relocation to new habitats. when the temperature in the atmosphere increases to when the sea level rises. An illustration of this inertia is the reduction in the number of glaciers in certain areas. There are about a third less glaciers than 135 years ago. This reduction isn't, however, simply the result of global warming but rather because the Earth is still returning to its normal state after the last Ice Age! Because the climate system is so slow, the climate will continue to warm up even after greenhouse gas emissions are stabilized or reduced. Sea level will also continue to rise for many hundreds of years after CO 2 emissions are stabilized. The slowness also adds to the uncertainties in our knowledge about climate change. Both the degree to which people add to the greenhouse effect and the possible consequences of human actions are difficult to study because it takes so long from when a gas is emitted to when we can measure the changes. When scientists try to predict the future, it takes a long time from when they make the calculations to when they can check to see if they were right. ESPERE Climate Encyclopaedia – www.espere.net - People changing the Climate More - page 3 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics 1. SLOW RESPONSE: This figure shows what might happen if man-made emissions of CO2 stop growing at some point during the next hundred years, and then start falling. After CO 2 emissions are reduced and concentrations in the atmosphere stabilize, surface air temperatures continue to rise slowly for a century or more. Expansion of the ocean due to warmer water continues long after CO2 emissions have been reduced and melting of ice sheets continues to contribute to sea-level rise for many centuries. Source: IPCC The slowness also means that the consequences of our actions are less noticeable now than they will be in the future. Emissions from one generation are most likely to affect future generations. Likewise, anything we do to slow down climate change now will only show results in the future. Part 4: Feedback effects When the Earth warms up, a large number of changes take place in the atmosphere, the oceans and on the land surface. Some of these changes can, in turn, affect the temperature. These are called feedback effects. Some of these feedback effects increase global warming, while others reduce it. Feedback from water vapor Water vapor is one of the most important feedback effects. A slight warming of the Earth due to more sunlight or an increased greenhouse effect, will lead to an increase in the amount of water vapor in the atmosphere. As water vapor is also a greenhouse gas, the extra water vapor will increase the greenhouse effect even more, leading to even greater warming. Thus water vapor has an amplifying effect on global warming. ESPERE Climate Encyclopaedia – www.espere.net - People changing the Climate More - page 4 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics Feedback from snow and ice cover The feedback effects from ice and snow-covered surfaces are similar. When the climate is cold, there is a lot of ice and snow on Earth. These shiny surfaces reflect sunlight away from the ground and make it even colder. A warmer climate means less ice and snow. This leads to less reflection of solar radiation to outer space and increased warming. 1. Ice cover in the Arctic ocean around Greenland. Photo: NASA. Feedback from clouds 2. Clouds. Photo: The NOAA Photo Library Another important feedback mechanism is changes in the cloud cover. When it gets warmer on Earth, the amount of water vapor in the atmosphere increases and more clouds may be formed. This can either increase or decrease warming, depending on what type of clouds they are. All clouds both cool the Earth by reflecting sunlight back into space and warm it up by absorbing heat from the surface in the same way that greenhouse gases do. Thin cirrus clouds (which appear high up in the atmosphere when the weather is fine) generally have a warming effect. Low cumulus and stratus clouds, on the other hand, have a cooling effect. However, we still don't know much about how climate change will affect the formation of different cloud types. Part 5: Abrupt changes The geological history of the Earth shows that there have been many abrupt changes in climate in the past. During the last Ice Age, sudden climate changes occurred about every thousand years. Scientists have drilled deep into the ice on Greenland and have analyzed layers of ice that have been there for tens of thousands of years. The analyses show that the average temperature on Greenland changed several times by 8 to16 oC over a short period of time – as little as a decade or two! The climate has been far more stable since the Ice Age ended and there have only been moderate climate variations since. These include the Little Ice Age in Europe between 1400 and 1850 when temperatures where much lower than average. Sources of abrupt changes ESPERE Climate Encyclopaedia – www.espere.net - People changing the Climate More - page 5 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics A gradual warming of the Earth – for example, due to stronger solar radiation or an increased greenhouse effect – can lead to abrupt changes in the climate system when a threshold is reached. For example, the abrupt climate changes during the last Ice Age probably occurred when the large inflows of freshwater from melting glaciers stopped flowing into the oceans changing ocean circulation patterns. Scientists think it unlikely that we will see dramatic changes in ocean currents in the future but they cannot rule out the possibility that the strength of the ocean currents will change quickly and lead to rapid climate changes in Europe. Another possible source of abrupt climate change is the enormous amounts of the greenhouse gas methane (CH4) trapped in the frozen ground in the Arctic. If global warming causes the permafrost to thaw and the methane to be released, this can lead to very rapid warming. 1. Abrupt changes: Reconstructed summer temperature in the Scandes Mountains, Sweden over the last 10,000 years. The curve shows a quite abrupt cooling episode that took place approximately 8,200 years ago. This event is also seen in temperature reconstructions from other locations in Europe and in ice cores from Greenland. It was probably caused by shifts in the ocean currents, caused by huge amounts of freshwater which were released to the oceans when the melting ice caps still left over North America after the Ice Age suddenly released huge amounts of water that had been trapped in lakes behind the ice. The temperature reconstruction was possible because scientists know what temperatures pine trees need to grow in the Scandes mountains. From plant remains, the scientist determined the maximum height above sea level where pine trees could grow at various times. A high limit for pine trees means a relatively warm climate. Source: Dahl and Nesje, The Holocene 6(4) 381-398 Modelling abrupt changes Climate models are best able to estimate gradual changes in climate resulting from higher concentrations of greenhouse gases and are often unable to predict abrupt changes. Calculating the likelihood and consequences of abrupt climate changes is very uncertain – partly because we do not know exactly where the “thresholds” lie, or what causes the abrupt changes. As a result we know little about when, where and how abrupt climate changes resulting from a warmer atmosphere will occur. Consequences of abrupt changes Sudden and unexpected climate changes often have serious consequences. Abrupt changes do not allow us the time or opportunity to prepare. For animal and plant life, abrupt changes are particularly serious, especially for species that, for example, have long lifetimes, are not very mobile or are specially adapted to one habitat. Sudden climate changes can give such species little time to find new homes and they might, therefore, face extinction. ESPERE Climate Encyclopaedia – www.espere.net - People changing the Climate More - page 6 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics