Long-term climate change
The best evidence of long-term climate change comes from Greenland and Antartic ice cores. Cores removed from ice sheets reveal layers going down through the ice. Rather like tree rings, each layer records a season of snowfall, buried and compressed by later falls.
The 3,200 metre East Antartic core records the climate of the last 800, 000 years. Air bubbles trapped in the ice contain atmospheric carbon dioxide and the ice itself preserves a record of oxygen isotopes. Low concentrations of carbon dioxide occur naturally during glacial (cold) periods, and high concentrations during interglacial (warm) periods. It is clear that atmospheric carbon dioxide levels are higher now (in the Holocene interglacial) that at any time for over a million years.
Climate: The average conditions of precpitation, temperature, pressure and wind measured over a 30-year period.
Climate change: Any long-term trend or shift in climate deteced by a sustained shift in the average value for any climate element (e.g. rainfall, drought, storminess).
Climatic change can be assessed across short, medium and long timescales. Short-term climate has been measured over the last few decades using sensitive, accurate equipment such as satellite and ocean temperature buoys. The medium-term (historical) timescales covers changes over the last few thousand years. Since around 1850, direct measurements of climate variables have been made using thermometers and rain gauges, but prior to this most climate data come from proxies, which indicate climate but do not directly measure it. Long-term climate change has occurred over several hundreds of thousands to millions of years. Evidence for this most often comes from ice cores.
The world's ocean play a key role in climatic regulation. Oceans act as carbon dioxide sinks, removing carbon dioxide from the atmosphere. Evaporation from oceans is vital for cloud formation, and maintaining precipitation levels. Ocean currents transfer warm water from the equator towards the poles, and this 'evens out' temperature extremes between the poles and the equator. Surface currents are driven by winds, as part of the global circulation. Deep ocean currents form the thermohaline circulation which flows between the oceans.
Thermohaline circulation is a global system of surface and deep-water ocean currents, driven by differences in temperatures and salinity between areas of the oceans.
Ocean currents, especially the role of the Gulf Stream and North Atlantic Drift, have recently become an important part of the climate change debate. The 2004 film The Day After Tomorrow portrayed the northern hemisphere plunged into an ice age as the North Atlantic Drift 'turned off' and its warming influence ceased. There is at least some science behind this.
Medium-term climate change
Historical and palaeo-environmental evidence
Proxy records are used to reconstruct climate before the start of instrumental records. These include paintings, poems, record books, diaries and journals which record weather at the time. The Thames froze over regularly between 1500 and 1850.
Since the mid-fourteenth century, the date of the grape harvest in the Burgundy region of France has been scrupulously recorded. This historical sequence of grape ripening dates is a proxy record, and has been to indicate past climate.
Little Ice Age: A cool period in Europe, in which many Alpine glaciers advanced. It lasted from around 1400 until 1850.
Medieval Warm Period: A period unusually warm North Atlantic climate lasting around 800 to 1400.
Proxy records need to be used with care. The date of the grape harvest could have been affected by non-climate factors such as conflict or diseased vines. It may also be hard to to know the relative importance of temperature, precipitation, sunlight hours or a combination of factors in influencing grape harvest dates.
Recent climate change
The instrumental record
Instrumental records from weather stations exist for the last 100 years or so. They show that near-surface air temperatures 0.74C between 1900 and 2000. The warming trend has been almost constant since 1960, and 11 of the world's 12 hottest years since 1850 occurred in the decade 1995 - 2006. The oceans have warmed to depths of 3,000 m. Warmer oceans cause problems for temperature-sensitive organisms such as coral. There is growing concern about ocean acidification. Measurements suggest that the pH of the oceans have decreased from 8.25 to 8.14 since 1750. The most likely cause is increased levels of dissolved carbon dioxide.
The instrumental record also demonstrates that global sea level has risen. Between 1961 and 2003 it rose by 1.88 mm per year, rising to 3.1 mm per year between 1993 and 2003. Most of this rise is attributed to thermal expansion, with water from melting glaciers and ice caps, so far, having a lesser impact on sea level.
Thermal expansion: The increased volume of the oceans as a result of their higher water temperature, leading to sea-level rise. It accounted for about 60% sea-level rise in the late twentieth century.
Recent climate change
Ice is a key indicator for climate scientists. Ice is found in many forms- as valley glaciers in the Alps, as ice caps on mountain ranges, as ice sheets in Greenland and Antarctica, as floating ice shelves, and as sea that forms in winter in high latitudes. In a warming world, ice might be expected to melt.
Drivers of climate change
There is no single cause of climate change. On the very long timescales of glacial-interglacial cycles, the most common explanation is the variation in the Earth's orbit around the sun. On timescales of hundreds to thousands of years, variations in the sun's solar output may fit observed climate trends. The warming that the Earth has experienced in the last few decades (global warming) is increasingly seen as driven by human pollution of the atmosphere. There is also evidence that volcanic activity can alter climate, but usually only for a few years.
Milutin Milankovich developed the theory of astronomical climate forcing. He argued that the surface temperature of the Earth changes over time because the Earth's orbit and axis tilt vary over time. These variations lead to changes in the amount and distribution of solar radiation received by the Earth from the sun. Over a timescale, of 100,000 years, the Earth's orbit changes from circular to elliptical and back again. This changes the amount of radiation received from the sun. On a timescale 41,000 years, the Earth's axis tilts from 21.5 to 24.5 and back again. This changes the seasonality of the Earth's climate. The smaller the tilt, the smaller the difference between summer and winter.
In support of Milankovitch's theory is the fact that ice ages (glacials) have occurred at regular 100,000-year intervals. However the actual impact of orbital changes on solar radiation amount and distribution is small. Many scientists agree that Milankovitch cycles may have been just enough to trigger a major global climate change, but that climate feedback mechanisms are needed to sustain it.
Drivers of climate change
Feedback effects are those that can either amplify a small change and make it larger (positive feedback), or diminish the change and make it smaller (negative feedback).
An example of positive feedback is snow and ice cover. Small icnreases in snow and ice dramatically raise surface albedo, so more solar energy is reflected back into space. This contributes to further cooling, which might encourage further snowfall.This may be how the 0.5C cooling identified by Milankovitch is amplified into a 5C global cooling.
An example of negative feedback is cloud cover. As global warming occurs, more evaporation will occur and this may increase global cloud cover. Increasingly cloudy skies could reflect more solar energy back into space, and dminish the effect of the warming.
The amount of energy emitted by the sun varies as a result of sunspots. These are dark spots that appear on the sun's surface, caused by intense magnetic storms. The effect of sunspots is to blast more solar radiation towards the Earth. There is a well-known 11 year-year sunspot cycle, as well as longer cycles, The total variation in solar radiation caused by sunspots is about 0.1%. Sunspots have been recored for around 2,000 years and there is a good record for around 400 years.
A long period with no sunspots, known as the Maunder Minimum, occured between 1645 and 1715, and this is often linked to the little ice age.
Drivers of climate change
Volcanic and cosmic causes
Volcanoes can impact climate change. During major explosive eruptions huge amounts of volcanic gas, aerosol droplets, and ash are injected into the stratosphere. Injected ash falls rapidly fall from the stratosphere -- most of it is removed within several days to weeks -- and has little impact on climate change. But volcanic gases like sulfur dioxide can cause global cooling, while volcanic carbon dioxide, a greenhouse gas, has the potential to promote global warming.
Fossil fuel use, as well as producing greenhouse gases, creates other by-products. These by-products are also pollutants, such as sulphur dioxide, soot, and ash. These pollutants however, also change the properties of clouds.Clouds are formed when water droplets are seeded by air-borne particles, such as pollen. Polluted air results in clouds with larger number of droplets than unpolluted clouds. This then makes those clouds more reflective. More of the sun’s heat and energy is therefore reflected back into space.This reduction of heat reaching the earth is known as Global Dimming.