* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download How Do We Know that Humans Are the Major Cause of Global
German Climate Action Plan 2050 wikipedia , lookup
Climate-friendly gardening wikipedia , lookup
Climate resilience wikipedia , lookup
Climate change mitigation wikipedia , lookup
Atmospheric model wikipedia , lookup
ExxonMobil climate change controversy wikipedia , lookup
Climate change adaptation wikipedia , lookup
Heaven and Earth (book) wikipedia , lookup
Climate change denial wikipedia , lookup
Climatic Research Unit documents wikipedia , lookup
Climate change in Tuvalu wikipedia , lookup
Economics of global warming wikipedia , lookup
Global warming controversy wikipedia , lookup
Effects of global warming on human health wikipedia , lookup
Global warming hiatus wikipedia , lookup
Low-carbon economy wikipedia , lookup
Climate governance wikipedia , lookup
Climate change and agriculture wikipedia , lookup
Media coverage of global warming wikipedia , lookup
Climate sensitivity wikipedia , lookup
Fred Singer wikipedia , lookup
Instrumental temperature record wikipedia , lookup
Mitigation of global warming in Australia wikipedia , lookup
Climate engineering wikipedia , lookup
Effects of global warming on humans wikipedia , lookup
Carbon Pollution Reduction Scheme wikipedia , lookup
Global Energy and Water Cycle Experiment wikipedia , lookup
Public opinion on global warming wikipedia , lookup
Climate change in the United States wikipedia , lookup
Citizens' Climate Lobby wikipedia , lookup
Global warming wikipedia , lookup
Scientific opinion on climate change wikipedia , lookup
Climate change and poverty wikipedia , lookup
Effects of global warming on Australia wikipedia , lookup
Climate change, industry and society wikipedia , lookup
Surveys of scientists' views on climate change wikipedia , lookup
Politics of global warming wikipedia , lookup
General circulation model wikipedia , lookup
Solar radiation management wikipedia , lookup
Climate change feedback wikipedia , lookup
Business action on climate change wikipedia , lookup
How Do We Know that Humans Are the Major Cause of Global Warming? http://www.ucsusa.org/global_warming/science_and_impa cts/science/human-contribution-to-gwfaq.html#.VqY46lK3vGs How do we know that humans are the major cause of global warming? The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states: it is a greater than a 90 percent certainty that emissions of heat-trapping gases from human activities have caused “most of the observed increase in globally averaged temperatures since the mid-20th century.”[1] We all know that warming— and cooling—has happened in the past, and long before humans were around. Many factors (called “climate drivers”) can influence Earth’s climate— such as changes in the sun’s intensity and volcanic eruptions, as well as heat-trapping gases in the atmosphere. So how do scientists know that today’s warming is primarily caused by humans putting too much carbon in the atmosphere when we burn coal, oil, and gas or cut down forests? There are human fingerprints on carbon overload. When humans burn coal, oil and gas (fossil fuels) to generate electricity or drive our cars, carbon dioxide is released into the atmosphere, where it traps heat. A carbon molecule that comes from fossil fuels and deforestation is “lighter” than the combined signal of those from other sources. As scientists measure the “weight” of carbon in the atmosphere over time they see a clear increase in the lighter molecules from fossil fuel and deforestation sources that correspond closely to the known trend in emissions.[2,3] Natural changes alone can’t explain the temperature changes we’ve seen. For a computer model to accurately project the future climate, scientists must first ensure that it accurately reproduces observed temperature changes. When the models include only recorded natural climate drivers—such as the sun’s intensity—the models cannot accurately reproduce the observed warming of the past half century. When human-induced climate drivers are also included in the models, then they accurately capture recent temperature increases in the atmosphere and in the oceans.[4,5,6] When all the natural and human-induced climate drivers are compared to one another, the dramatic accumulation of carbon from human sources is by far the largest climate change driver over the past half century. Lower-level atmosphere—which contains the carbon load—is expanding. The boundary between the lower atmosphere (troposphere) and the higher atmosphere (stratosphere) has shifted upward in recent decades. See the ozone FAQ for a figure illustrating the layers of the atmosphere.[6,7,8] This boundary has likely changed because heat-trapping gases accumulate in the lower atmosphere and that atmospheric layer expands as it heats up (much like warming the air in a balloon). And because less heat is escaping into the higher atmosphere, it is likely cooling. This differential would not occur if the sun was the sole climate driver, as solar changes would warm both atmospheric layers, and certainly would not have warmed one while cooling the other. Direct evidence of human contribution to atmospheric CO2 Carbon dioxide (CO2) is the main heat-trapping gas largely responsible for most of the average warming over the past several decades.[2] To compare how CO2 stacks up in influence to the many other important heat-trapping gases contributing to climate change see the CO2 FAQ. There is a way that scientists can tease apart the atmospheric concentration of CO2 to see how much of the CO2 is from natural sources and how much is from combusted fossil fuel sources. The atmospheric concentration of CO2 has increased from a pre-industrial era (AD 1000 – 1750) concentration of approximately 280 parts per million (ppm) to around 383 ppm, as measured at Mauna Loa, Hawaii in 2007.[2,9] The carbon in the atmospheric CO2 contains information about its source, so that scientists can tell that fossil fuel emissions comprise the largest source of the increase since the pre-industrial era. Here’s how scientists know. The same elements (i.e. same number of protons in the nucleus) with different mass numbers (arising from the different numbers of neutrons in the nucleus) are called isotopes. Each carbon molecule has six protons in the nucleus, but there are many different isotopes with varying numbers of neutrons in the nucleus.[10] Carbon isotopes from different sources are “lighter” (high negative value) or heavier (lower negative value). For example, carbon from ocean is the standard with a value of “0” while carbon from fossil fuels ranges from -20 to -32.[11] While atmospheric carbon has an average value of -5 to -9, it is becoming “lighter” over time as carbon from fossil fuels become more abundant in the atmosphere (Figure 1).[9,11,12] Figure 1. Direct Evidence of Fossil Fuel Derived CO2 in the Atmosphere The combination of natural drivers plus human drivers best match reality (1) Natural and Human Factors that Influence the Climate (known as “climate drivers”) Many natural and human factors influence climate. Natural factors include the energy from the sun; periodic volcanic eruptions of tiny particles, dust, and salt spray—all known as aerosols— many that can reflect sunlight; and natural carbon cycle processes such as termite mounds in Africa that emit methane or tiny organisms in the ocean surface that take up carbon dioxide. Human climate drivers include heat-trapping emissions from burning coal, gas and oil in power plants and cars; cutting down and burning forests; tiny pollution particles known as aerosols; black carbon pollution more commonly referred to as soot; and changes in land use that change how much the Earth’s surface reflects the sun’s energy back into space (referred to as albedo). Some of these climate drivers result in net warming and others lead to cooling, but all are usually expressed as Radiative Forcing (RF) in units of watts per square meter. When all the natural and human-induced climate drivers are stacked up and compared to one another, the accumulation of human-released heat-trapping gases in the atmosphere is so large that it has very likely swamped other climate drivers over the past half century, leading to observed global warming (see Figure 2).[2,4,5] Figure 2. Twentieth Century History of Climate Drivers Heat-trapping emissions (greenhouse gases) far outweigh the effects of other drivers acting on Earth’s climate. Source: Hansen et al. 2005, Figure adapted by Union of Concerned Scientists [5] (2) Cooling that partially offsets recent warming: “Global Dimming” only natural climate drivers do not recreate the observed trends very well. Figure 3. Combined Natural Plus Human Drivers Best Match Observed Atmospheric Temperature The figure above also depicts the sharp cooling influence a large volcanic eruption can have as it spews tiny particles high into the stratosphere (the layer of the atmosphere above the troposphere where weather typically occurs). The massive explosions from Krakatoa (Indonesia) in 1883 and Mount Pinatubo (Philippines) in 1991, for example, can be seen as the two largest downward spikes in the blue volcanic data depicted in the figure. These particles prevented the full energy of the sun from reaching the surface of Earth and thus created a cooling trend for several years. Fossil fuel burning by humans also emits tiny particles. Some particles reflect sunlight back to space (aerosols), similar to the volcanic particles; other pollution particles such as soot (black carbon) absorb the sunlight, which leads to local warming of the atmosphere level where the soot particles circulate (see Aerosols FAQ). Both types of humancreated particles lead to a decrease in the amount of sun’s energy reaching the surface of Earth. The term “global dimming” has been used to describe this phenomenon. There very likely would have been even more warming in the past 60 years if it were not for these human-made and natural tiny particles.[5] (3) Observed temperature changes on land and ocean compared with natural and human climate drivers The IPCC has carefully documented observed changes in air temperature, ocean temperature, ice retreat, and sea level rise over the past century. These observed changes are then compared in climate computer models with natural climate drivers and human climate drivers. The IPCC concluded that the observed changes are unlikely to be the result solely of natural processes. [4,6] Figures 3 and 4 illustrate several examples from climate models that show that the combination of natural and human climate drivers closely match observed historical trends. Conversely, the corresponding figures from climate models using Figure 4. Combined Natural Plus Human Drivers Best Match Observed Ocean Temperature The sky is rising! The boundary between the lower atmosphere (troposphere) and the upper atmosphere (stratosphere) has shifted upward in recent decades. This boundary is called the tropopause. Similar to the cases outlined above, models that have both natural and human climate drivers match the observed change in tropopause height better than models with only natural climate drivers. The likely cause of the rise in tropopause height is from heat-trapping gases accumulating in and heating up the troposphere and conversely blocking heat from getting into the stratosphere, thus causing cooling there.[6,7,8] Decreased ozone in the stratosphere also adds to this cooling stratosphere trend (see ozone FAQ.) This would not occur if the sun was the sole climate driver, as solar changes would have warmed both the stratosphere and the troposphere. One way to think about this is how when air of a hot air balloon heats up it expands and the top of the balloon rises. The same general idea can be applied here—the troposphere volume expands and rises as it warms, since the boundary between the troposphere and the stratosphere is in part defined by a change in temperature. http://ncse.com/climate/climate-change-101/howmuch-human-responsibility-for-climate-change How Much Does Human Activity Affect Climate Change? January 5th, 2012 The Earth’s climate is changing rapidly. Scientists trying to find out what’s causing climate change work like detectives, gathering evidence to rule out some suspects and to ascertain just who is responsible. It’s clear, based on over a century of scientific investigation, that humans are responsible for most of the climate change we’ve seen over the last 150 years. Humans are not the only suspects. The climate has changed throughout the Earth’s history, well before humans evolved. The Sun is the primary driver of the climate. Roughly speaking, global temperatures rise when more energy from the Sun enters the atmosphere than returns to space through the atmosphere. The climate cools any time more energy returns to space than comes in from the Sun. While humans can influence that balance, other factors, from continental drift and changes in the shape of the Earth’s orbit to variations in the Sun’s activity and phenomena like El Niño, can all influence the climate. Considering the pace of climate change today, scientists can rule out most of those suspects: some happen too slowly to explain current climate change, while others move in small cycles, not long trends, and others only influence the climate in part of the planet. Scientists know about these factors and can account for them when assessing human-caused climate change. Carbon dioxide is rising because of human actions: Scientists can measure the increase of atmospheric carbon dioxide over the last 150 years. By comparing the type of carbon being added to the atmosphere, they see that the kind of carbon released by burning coal, gasoline, and natural gas is diluting the naturally-occurring carbon dioxide in the atmosphere. Data from Trudinger, et al (1999). Image by Steve Newton for NCSE, in 2012. The potential for human beings to alter climate was first proposed over a century ago, building on research in the 1850s by John Tyndall. Light from the Sun warms the Earth’s surface, which then gives off energy as infrared radiation, the heat you feel from asphalt on a sunny day. Greenhouse gases such as water vapor and carbon dioxide (CO2), absorb this radiating energy, heating the atmosphere and the surface. This process results in the Earth’s temperature being warmer than it would if it were heated only by direct sunlight. For over 100 years, scientists have regarded humans as the prime suspect in current climate changes. Around the turn of the 20th century, Svante Arrhenius was the first to suggest that people could, through the burning of coal, increase the amount of greenhouse gases in the atmosphere and amplify the natural warming effect, thereby causing the atmosphere to warm more than it would through strictly natural processes. When humans burn gasoline, coal, natural gas, and other common fuels to make electricity or drive cars, they release a substantial amount of carbon dioxide into the atmosphere. For every gallon (or liter) of gasoline your car burns, 1300 times that volume of CO2 is released (a gallon of gas weighs about 6 pounds or 2.8 kilograms, but the released CO2 would weigh over 19 pounds or 8.75 kilograms). Greenhouse gases are emitted from power plants and cars, but also from landfills, from farms and cleared forests, and through other subtle processes. An interactive map from the Environmental Protection Agency shows US sources of key greenhouse gases. The World Resources Institute cataloged (PDF) global sources of greenhouse gases in 2005. In the 1950s, scientists began methodically measuring global increases in carbon dioxide. Since then they’ve been able to confirm that the increase has been caused primarily from the burning of fossil fuels (and through other human activities, such as clearing land, as well). This increase, and changes in the type of CO2 being added to the atmosphere provide the “smoking gun” that shows that humans are responsible for the increased levels of carbon dioxide in the atmosphere. Modeling the Climate System To clinch the case that climate change is mostly caused by humans, scientists had to take into account other factors: complicated atmospheric physics, the interactions between air and land and between air and water, changing amounts of ice and of desert and forest, and the natural processes that have changed the climate for 4.54 billion years. To do all this, scientists recreate the crime scene. Because there’s only one Earth, they do that with computers. Climate scientists use powerful computers to construct models based on physics of the climate system. These models enable scientists to make predictions and test hypotheses about what processes affect the climate. Modeling the flow of water through the ocean: To model climate change, scientists incorporate complex models of the fluid dynamics of the ocean and atmosphere, such as this image from a model of the ocean around Africa. In constructing such models, climate scientists start with basic science: thermodynamic principles, orbital dynamics, interactions between infrared radiation and carbon dioxide and other gases, and other such factors influencing the basic balance of heat entering and leaving the atmosphere. They combine these with actual measurements of data, such as the current concentration of greenhouse gases in the atmosphere. And they test hypotheses about which processes affect which phenomena. They can then see what the resultant models predict about the climate system. When the predictions match observations, it counts as a confirmation of the model and its underlying assumptions. When the predictions diverge from what is observed, climate scientists revise their hypotheses. By such a trialand-error process, they are able to ensure that their hypotheses become more and more accurate and reliable. Such models can be applied to the Earth as a whole or to different regions of the planet. To see how climate change affects different parts of the Earth, climate scientists break the models down into smaller pieces, calculate how small parts of the Earth’s surface react to the Sun and greenhouse gases, and then connect those small parts together, based on measurements of how the atmosphere and ocean interact. To ensure the accuracy of the models at projecting future climate trends, the models are often run backwards in time to “retrodict” past climate changes, and then compared with paleoclimate observations. The models through this process have become remarkably accurate and give the climate research community confidence that the future projections are robust. Scientists can do experiments with these models that they can’t do on the planet. They can set the atmosphere to match conditions a century ago, and see whether the model’s predictions match what scientists measured at the time. And they can set the models to match conditions millions of years ago, to better understand how past climate changed. This allows them to verify that the models are accurate and to fine-tune the output. They can also remove the effects of human activities from the models, and see how much of the climate change they observe is still predicted by the models. These models, which are carefully vetted by other experts, agree that human activities, especially the burning of fossil fuels and resulting release of carbon dioxide into the atmosphere, are now substantially altering the planet’s climate, as summarized by the Intergovernmental Panel on Climate Change. In many different sorts of climate models, from teams competing to produce the models that most accurately predict past climates, the consistent result is that humans are producing most of the climate change that we've observed in the last century. One recent analysis, reported in Scientific American, estimated that at least three quarters of climate change since 1950 is due to human activities.