Global opvarmning og klimaforandringer

Global warming and climate change are often used interchangeably. But, more accurately, global warming is the mainly human-caused increase in global surface temperatures and its projected continuation, while climate change includes both global warming and its effects, such as changes in precipitation. While there have been prehistoric periods of global warming, observed changes since the mid-20th century have been unprecedented in rate and scale. The Intergovernmental Panel on Climate Change Fifth Assessment Report concluded, "It is that human influence has been the dominant cause of the observed warming since the mid-20th century". The largest human influence has been the emission of greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Fossil fuel burning is the dominant source of these gases, with agricultural emissions and deforestation also playing significant roles. and are not disputed by any scientific body of national or international standing. The effects of global warming include rising sea levels, regional changes in precipitation, more frequent extreme weather events such as heat waves, and expansion of deserts. Surface temperature increases are greatest in the Arctic, which have contributed to the retreat of glaciers, permafrost, and sea ice. Overall, higher temperatures bring more rain and snowfall, but for some regions droughts and wildfires increase instead. Climate change threatens to diminish crop yields, harming food security, and rising sea levels may flood coastal infrastructure and force the abandonment of many coastal cities. Environmental impacts include the extinction or relocation of many species as their ecosystems change, most immediately in coral reefs, mountains, and the Arctic. Some impacts, such as loss of snow cover, increased water vapour, and melting permafrost, cause feedback effects that further increase the rate of global warming. Although the parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below in the Paris Agreement of 2016, the Earth's average surface temperature has already increased by about half this threshold. With current policies and pledges, global warming by the end of the century is expected to reach just over 2 °C to 4 °C, depending on how sensitive the climate is to emissions. The IPCC has stressed the need to keep global warming below 1.5 °C compared to pre-industrial levels in order to avoid irreversible impacts. At the current greenhouse gas emission rate of 42 gigatons per year, the carbon budget for staying below 1.5°C would be exhausted by 2028. Observed temperature rise Climate proxy records show that natural variations offset the early effects of the Industrial Revolution, so there was little net warming between the 18th century and the mid-19th century, when thermometer records began to provide global coverage. The Intergovernmental Panel on Climate Change has adopted the baseline reference period 1850–1900 as an approximation of pre-industrial global mean surface temperature. Currently, surface temperatures are rising by about 0.2 °C per decade. Since 1950, the number of cold days and nights have decreased, and the number of warm days and nights have increased. Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the Little Ice Age, were not as synchronous as current warming, but may have reached temperatures as high as those of the late-20th century in a limited set of regions. Past examples of climate change provide insight into modern climate change. Although the most common measure of global warming is the increase in the near-surface atmospheric temperature, over 90% of the additional energy stored in the climate system over the last 50 years has warmed ocean water. The remainder of the additional energy has melted ice and warmed the continents and the atmosphere. The warming evident in the instrumental temperature record is consistent with a wide range of observations, documented by many independent scientific groups; for example, in most continental regions the frequency and intensity of heavy precipitation has increased. Further examples include sea level rise, widespread melting of snow and land ice, increased heat content of the oceans, increased humidity, and the earlier timing of spring events, such as the flowering of plants. Regional trends Global warming refers to global averages, with the amount of warming varying by region. Since the pre-industrial period, global average land temperatures have increased almost twice as fast as global average temperatures. This is due to the larger heat capacity of oceans and because oceans lose more heat by evaporation. Patterns of warming are independent of the locations of greenhouse gas emissions because the gases persist long enough to diffuse across the planet; however, localized black carbon deposits on snow and ice do contribute to Arctic warming. The Northern Hemisphere and North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more snow area and sea ice, because of how the land masses are arranged around the Arctic Ocean. As these surfaces flip from being reflective to dark after the ice has melted, they start absorbing more heat. The Southern Hemisphere already had little sea ice in summer before it started warming. Arctic temperatures have increased and are predicted to continue to increase during this century at over twice the rate of the rest of the world. As the temperature difference between the Arctic and the equator decreases, ocean currents that are driven by that temperature difference, like the Gulf Stream, weaken. Short-term slowdowns and surges Because the climate system has large thermal inertia, it can take centuries for the climate to fully adjust. While record-breaking years attract considerable public interest, individual years are less significant than the overall trend. Global surface temperature is subject to short-term fluctuations that overlie long-term trends, and can temporarily mask or magnify them. An example of such an episode is the slower rate of surface temperature increase from 1998 to 2012, which was dubbed the global warming hiatus. Throughout this period, ocean heat storage continued to progress steadily upwards, and in subsequent years, surface temperatures have spiked upwards. The slower pace of warming can be attributed to a combination of natural fluctuations, reduced solar activity, and increased reflection sunlight of by particles from volcanic eruptions. Physical drivers of recent climate change By itself, the climate system experiences various cycles which can last for years to decades or centuries. Other changes are caused by an imbalance of energy at the top of the atmosphere: external forcings. These forcings are "external" to the climate system, but not always external to the Earth. Examples of external forcings include changes in the composition of the atmosphere, solar luminosity, volcanic eruptions, and variations in the Earth's orbit around the Sun. Attribution of climate change is the effort to scientifically show which mechanisms are responsible for observed changes in Earth's climate. First, known internal climate variability and natural external forcings need to be ruled out. Therefore, a key approach is to use computer modelling of the climate system to determine unique "fingerprints" for all potential causes. By comparing these fingerprints with observed patterns and evolution of climate change, and the observed history of the forcings, the causes of the observed changes can be determined. For example, solar forcing can be ruled out as major cause because its fingerprint is warming in the entire atmosphere, and only the lower atmosphere has warmed as expected for greenhouse gases. Greenhouse gases Greenhouse gases trap heat radiating from the Earth to space. This heat, in the form of infrared radiation, gets absorbed and emitted by these gases in the atmosphere, thus warming the lower atmosphere and the surface. Before the Industrial Revolution, naturally occurring amounts of greenhouse gases caused the air near the surface to be warmer by about than it would be in their absence. Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing temperature of water. While water vapour and clouds are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore considered feedbacks. Increased concentrations of gases such as, ozone and are external forcing on the other hand. Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels, has increased the amount of greenhouse gases in the atmosphere. This CO2, methane, tropospheric ozone, CFCs, and nitrous oxide has increased radiative forcing. As of 2011, the concentrations of CO2 and methane had increased by about 40% and 150%, respectively, since pre-industrial times. In 2013, CO2 readings taken at the world's primary benchmark site in Mauna Loa surpassing 400 ppm for the first time. These levels are much higher than at any time during the last 800,000 years, the period for which reliable data have been collected from ice cores. Less direct geological evidence indicates that CO2 values have not been this high for millions of years. Global anthropogenic greenhouse gas emissions in 2010 were equivalent to 49 billion tonnes of carbon dioxide . Of these emissions, 65% was carbon dioxide from fossil fuel burning and industry, 11% was carbon dioxide from land use change, which is primarily due to deforest.