Climate Change: A Summary Of The Science - Royal Society
Climate change:a summary of the scienceSeptember 2010
GlossaryWm-2 (watts per metre squared)The amount of energy that falls on a square metre in one second.Sometimes known as a flux.Carbon cycleThe term used to describe the flow of carbon, in its various forms,between the atmosphere, oceans, plants, soils and rocks. In theatmosphere, carbon mostly exists as carbon dioxide, but it exists indifferent forms in other components, such as organic carbon in the soil.Climate forcing (also known as radiative forcing)The imbalance in the Earth’s energy budget resulting from, for example,changes in the energy received from the Sun, changes in the amounts orcharacteristics of greenhouse gases and particles, or changes in the natureof the Earth’s surface. Hence climate forcing can result from both humanactivity and natural causes. It is given in units of Wm-2.Climate sensitivityThis is the amount of climate change (as measured by the change inglobally-averaged surface temperature) for a given amount of climateforcing. It is often quoted (as will be the case here) as the temperaturechange that eventually results from a doubling in CO2 concentrations,which is calculated to cause a climate forcing of about 3.6 Wm-2.Internal climate variability:Climate change that occurs in the absence of natural or human-inducedclimate forcing as a result of interactions within and between the variouscomponents of the climate system (such as the atmosphere, the oceansand the frozen world).
Introduction1Changes in climate have significant implications for present lives, for future generationsand for ecosystems on which humanity depends. Consequently, climate change hasbeen and continues to be the subject of intensive scientific research and public debate.2There is strong evidence that the warming of the Earth over the last half-century hasbeen caused largely by human activity, such as the burning of fossil fuels and changesin land use, including agriculture and deforestation. The size of future temperatureincreases and other aspects of climate change, especially at the regional scale, are stillsubject to uncertainty. Nevertheless, the risks associated with some of these changesare substantial. It is important that decision makers have access to climate science ofthe highest quality, and can take account of its findings in formulating appropriateresponses.3In view of the ongoing public and political debates about climate change, the aim of thisdocument is to summarise the current scientific evidence on climate change and itsdrivers. It lays out clearly where the science is well established, where there is wideconsensus but continuing debate, and where there remains substantial uncertainty. Theimpacts of climate change, as distinct from the causes, are not considered here. Thisdocument draws upon recent evidence and builds on the Fourth Assessment Report ofWorking Group I of the Intergovernmental Panel on Climate Change (IPCC), published in2007, which is the most comprehensive source of climate science and its uncertainties.The Royal SocietyClimate change: a summary of the science I September 2010 I 1
Climate and climate change:some background scienceThe greenhouse effect4The Sun is the primary source of energy for the Earth’s climate. Satellite observationsshow that about 30% of the Sun’s energy that reaches the Earth is reflected back tospace by clouds, gases and small particles in the atmosphere, and by the Earth’ssurface. The remainder, about 240 Watts per square metre (Wm-2) when averaged overthe planet, is absorbed by the atmosphere and the surface.5To balance the absorption of 240 Wm-2 from the Sun, the Earth’s surface andatmosphere must emit the same amount of energy into space; they do so as infraredradiation. On average the surface emits significantly more than 240 Wm-2, but the neteffect of absorption and emission of infrared radiation by atmospheric gases and cloudsis to reduce the amount reaching space until it approximately balances the incomingenergy from the Sun. The surface is thus kept warmer than it otherwise would bebecause, in addition to the energy it receives from the Sun, it also receives infraredenergy emitted by the atmosphere. The warming that results from this infrared energy isknown as the greenhouse effect.6Measurements from the surface, research aircraft and satellites, together withlaboratory observations and calculations, show that, in addition to clouds, the twogases making the largest contribution to the greenhouse effect are water vapourfollowed by carbon dioxide (CO2). There are smaller contributions from many othergases including ozone, methane, nitrous oxide and human-made gases such as CFCs(chlorofluorocarbons).Climate change7Climate change on a global scale, whether natural or due to human activity, can beinitiated by processes that modify either the amount of energy absorbed from the Sun,or the amount of infrared energy emitted to space.8Climate change can therefore be initiated by changes in the energy received from theSun, changes in the amounts or characteristics of greenhouse gases, particles andclouds, or changes in the reflectivity of the Earth’s surface. The imbalance between theabsorbed and emitted radiation that results from these changes will be referred to hereas “climate forcing” (sometimes known as “radiative forcing”) and given in units ofWm-2. A positive climate forcing will tend to cause a warming, and a negative forcing acooling. Climate changes act to restore the balance between the energy absorbed fromthe Sun and the infrared energy emitted into space.9In principle, changes in climate on a wide range of timescales can also arise fromvariations within the climate system due to, for example, interactions between theoceans and the atmosphere; in this document, this is referred to as “internal climateThe Royal SocietyClimate change: a summary of the science I September 2010 I 2
variability”. Such internal variability can occur because the climate is an example of achaotic system: one that can exhibit complex unpredictable internal variations even inthe absence of the climate forcings discussed in the previous paragraph.10There is very strong evidence to indicate that climate change has occurred on a widerange of different timescales from decades to many millions of years; human activity isa relatively recent addition to the list of potential causes of climate change.11The shifts between glacial and interglacial periods over the past few million years arethought to have been a response to changes in the characteristics of the Earth’s orbitaround the Sun. While these led to only small changes in the total energy received fromthe Sun, they led to significant changes in its geographical and seasonal distribution.The large changes in climate, in moving in and out of glacial periods, provide evidenceof the sensitivity of climate to changes in the Earth’s energy balance, whetherattributable to natural causes or to human activity.Mechanisms of global climate change12Once a climate forcing mechanism has initiated a climate response, this climate changecan lead to further changes; for example, in response to a warming, the amount ofwater vapour is expected to increase, the extent of snow and ice is expected todecrease, and the amount and properties of clouds could also change. Such changescan further modify the amount of energy absorbed from the Sun, or the amount ofenergy emitted by the Earth and its atmosphere, and lead to either a reduction oramplification of climate change.13The overall effect of the changes resulting from climate forcing determine a keycharacteristic of the climate system, known as the “climate sensitivity” – this is theamount of climate change (as measured by the equilibrium change in globally-averagedsurface temperature) caused by a given amount of climate forcing. It is often quoted (aswill be the case here) as the temperature change that eventually results from a doublingin CO2 concentrations since pre-industrial times, and is calculated to cause a climateforcing of about 3.6 Wm-2.14The nature of the climate system is determined by interactions between the movingatmosphere and oceans, the land surface, the living world and the frozen world. Therate at which heat is moved from the surface to the ocean depths is an important factorin determining the speed at which climate can change in response to climate forcing.15Since variations in climate can result from both climate forcing and internal climatevariability, the detection of forced climate change in observations is not alwaysstraightforward. Furthermore, the detection of climate change in observations, beyondthe expected internal climate variability, is not the same as the attribution of that changeThe Royal SocietyClimate change: a summary of the science I September 2010 I 3
to a particular cause or causes. Attribution requires additional evidence to provide aquantitative link between the proposed cause and the observed climate change.Modelling the climate system16Current understanding of the physics (and increasingly the chemistry and biology) ofthe climate system is represented in a mathematical form in climate models, which areused to simulate past climate and provide projections of possible future climate change.Climate models are also used to provide quantitative estimates to assist the attributionof observed climate change to a particular cause or causes.17Climate models vary considerably in complexity. The simplest can be described by a fewequations, and may represent the climate by global-average surface temperature alone.The most complicated and computer-intensive models represent many details of theinteractions between components of the climate system. These more complex modelsrepresent variations in parameters such as temperature, wind and humidity withlatitude, longitude and altitude in the atmosphere, and also represent similar variationsin the ocean. In complex climate models climate sensitivity emerges as an output; in thesimpler calculations it is specified either as an input or it emerges as a consequence ofsimplified (but plausible) assumptions.18By applying established laws of fluid dynamics and thermodynamics, the more complexclimate models simulate many important weather phenomena that determine theclimate. However, limitations of computer power mean that these models cannotdirectly represent phenomena occurring at small scales. For example, individual cloudsare represented by more approximate methods. Since there are various ways to makethese approximations, the representation can vary in climate models developed atdifferent climate institutes. The use of these different approximations leads to a range ofestimates of climate sensitivity, especially because of differences between models in theresponse of clouds to climate change. There are intensive efforts to compare themodels with observations and with each other. The spread of results from these modelsgives useful information on the degree of confidence in the reliability of projections ofclimate change.19Unlike weather-forecast models, climate models do not seek to predict the actualweather on a particular day at a particular location. The more complex models dohowever simulate individual weather phenomena, such as mid-latitude depressions andanticyclones, and aim to give simulations of possible weather sequences much fartherinto the future than weather forecast models. From such simulations, one can derivethe characteristics of climate likely to occur in future decades, including meantemperature and temperature extremes.The Royal SocietyClimate change: a summary of the science I September 2010 I 4
Aspects of climate change onwhich there is wide agreementChanges in global-average surface temperature20Measurements suitable for showing how surface temperature has changed with timeacross the world became available around 1850. Analyses of these data, in a number ofinstitutes, try to take into account changing distributions of measurements, changingobservation techniques, and changing surroundings of observing stations (e.g. somestations become more urban with time, which can make measurements from them lessrepresentative of wider areas).21Measurements show that averaged over the globe, the surface has warmed by about0.8oC (with an uncertainty of about 0.2oC) since 1850. This warming has not beengradual, but has been largely concentrated in two periods, from around 1910 to around1940 and from around 1975 to around 2000. The warming periods are found in threeindependent temperature records over land, over sea and in ocean surface water. Evenwithin these warming periods there has been considerable year-to-year variability. Thewarming has also not been geographically uniform – some regions, most markedly thehigh-latitude northern continents, have experienced greater warming; a few regionshave experienced little warming, or even a slight cooling.22When these surface temperatures are averaged over periods of a decade, to removesome of the year-to-year variability, each decade since the 1970s has been clearlywarmer (given known uncertainties) than the one immediately preceding it. The decade2000-2009 was, globally, around 0.15oC warmer than the decade 1990-1999.23Local temperatures are generally a poor guide to global conditions. For example, acolder-than-average winter in the UK does not mean that colder-than-averageconditions are experienced globally. Similarly, observed variations in global temperatureover a period of just a few years could be a misleading guide to underlying longer-termtrends in global temperature.Other changes in climate24Not all aspects of the climate system have been observed over as long a period assurface temperatures – for example, those based on satellite observations date back, atbest, to the 1970s. Nor are the measurements of other aspects of the climate systemalways of the same quality. Collectively, however, they provide much evidence ofclimate change consistent with the surface temperature changes. This includesincreases in the average temperature of both the upper 700m of the ocean and thetroposphere (the atmosphere up to 10-18km), widespread (though not universal)decreases in the length of mountain glaciers and increases in average sea level. Therehas been an overall decline in the area covered by sea-ice floating on the Arctic Oceanover the past 30 years (although there has been a small increase in the area covered bysea-ice around Antarctica).The Royal SocietyClimate change: a summary of the science I September 2010 I 5
Changes in atmospheric composition25Global-average CO2 concentrations have been observed to increase from levels ofaround 280 parts per million (ppm) in the mid-19th century to around 388 ppm by theend of 2009. CO2 concentrations can be measured in “ancient air” trapped in bubbles inice, deep below the surface in Antarctica and Greenland; these show that present-dayconcentrations are higher than any that have been observed in the past 800,000 years,when CO2 varied between about 180 and 300 ppm. Various lines of evidence pointstrongly to human activity being the main reason for the recent increase, mainly due tothe burning of fossil fuels (coal, oil, gas) with smaller contributions from land-usechanges and cement manufacture. The evidence includes the consistency betweencalculations of the emitted CO2 and that expected to have accumulated in theatmosphere, the analysis of the proportions of different CO2 isotopes, and the amountof oxygen in the air.26These observations show that about half of the CO2 emitted by human activity since theindustrial revolution has remained in the atmosphere. The remainder has been taken upby the oceans, soils and plants although the exact amount going to each of theseindividually is less well known.27Concentrations of many other greenhouse gases have increased. The concentration ofmethane has more than doubled in the past 150 years; this recent and rapid increase isunprecedented in the 800,000 year record and evidence strongly suggests that it arisesmainly as a result of human activity.Climate forcing by greenhouse gas changes28Changes in atmospheric composition resulting from human activity have enhanced thenatural greenhouse effect, causing a positive climate forcing. Calculations, which aresupported by laboratory and atmospheric measurements, indicate that these additionalgases have caused a climate forcing during the industrial era of around 2.9 Wm-2, withan uncertainty of about 0.2 Wm-2. Other climate change mechanisms resulting fromhuman activity are more uncertain (see later); calculations that take into account theseother positive and negative forcings (including the role of atmospheric particles) indicatethat the net effect of all human activity has caused a positive climate forcing of around1.6 Wm-2 with an estimated uncertainty of about 0.8 Wm-2.29Application of established physical principles shows that, even in the absence ofprocesses that amplify or reduce climate change (see paragraphs 12 & 13), the climatesensitivity would be around 1oC, for a doubling of CO2 concentrations. A climate forcingof 1.6 Wm-2 (see previous paragraph) would, in this hypothetical case, lead to a globallyaveraged surface warming of about 0.4oC. However, as will be discussed in paragraphThe Royal SocietyClimate change: a summary of the science I September 2010 I 6
36, it is expected that the actual change, after accounting for the additional processes,will be greater than this.Carbon dioxide and climate30Evidence from ice cores indicates an active role for CO2 in the climate system. This isbecause the amount of carbon held in oceans, soils and plants depends on temperatureand other conditions. In other words, changes in CO2 can lead to climate change andclimate change can also alter the concentrations of CO2.The Royal SocietyClimate change: a summary of the science I September 2010 I 7
Aspects of climate change wherethere is a wide consensus butcontinuing debate and discussionThe carbon cycle and climate31Once atmospheric CO2 concentrations are increased, carbon cycle models (whichsimulate the exchange of carbon between the atmosphere, oceans, soils and plants)indicate that it would take a very long time for that increased CO2 to disappear; this ismainly due to well-known chemical reactions in the ocean. Current understandingindicates that even if there was a complete cessation of emissions of CO2 today fromhuman activity, it would take several millennia for CO2 concentrations to return to preindustrial concentrations.Other drivers of global climate change32In addition to changes in greenhouse gas concentrations, there are a large number ofless well characterised contributions to climate forcing, both natural and humaninduced.33 Volcaniceruptions are examples of a natural climate forcing mechanism. An individualvolcanic eruption has its largest effects on the climate for only a few years after theeruption; these effects are dependent on the location, size and type of the eruption.34Natural forcing due to sustained variations in the energy emitted by the Sun over thepast 150 years is estimated to be small (about 0.12 Wm-2); however, direct observationsof the energy emitted by the Sun only became available in the 1970s and estimates overlonger periods rely on observations of changes in other characteristics of the Sun. Anumber of mechanisms have been proposed that could reduce or amplify the effect ofsolar variations; these remain areas of active research.35Human activity results in emissions of many short-lived gases (such as carbonmonoxide and sulphur dioxide) and particles into the atmosphere. These affect theatmospheric concentrations of other climate-important gases such as ozone, and otherparticles which lead to a climate forcing. Calculations, coupled to a variety ofatmospheric observations, indicate that particles have caused a negative
Climate change 7 Climate change on a global scale, whether natural or due to human activity, can be initiated by processes that modify either the amount of energy absorbed from the Sun, or the amount of infrared energy emitted to space. 8 Climate change can therefore be initiated by changes in the energy received from the
Food Security and Nutrition 1.1.Climate Change and Agriculture Climate change shows in different transformations of climate variables that are causing significant economic, social and environmental effects. The Intergovernmental Panel on Climate Change (IPCC), in 2002, has defined climate change as “any change in climate over time,
Gender and climate change – Women as agents of change. IUCN climate change briefing, December 2007 Gender, Climate Change and Human Security. Lessons from Bangladesh, Ghana and Senegal. Prepared for ELIAMEP for WEDO, May 2008 Gender and Climate Change. Gender in CARE’s Adaptation Learning Programme for Africa. CARE and Climate Change, 2011 –
Global warming is when Earth’s air and the water get warmer. Global warming is one part of climate change. This does not sound good! Climate Change in American Samoa You may have heard people talk about Climate Change or Global Warming. Do you know what these are? Uh-oh! 5 Fill in the blank spaces with words from the word bank: Climate change affects the climate of the entire _. Climate .
o Scientific basis of climate change o Impacts of climate change o Future risks of climate change o Options for adaptation to climate change o Mitigation of climate change 195 member countries Assessments written by hundredsof leading scientist from around the world Assessments reviewed by thousandsof experts (this is quality control)
International Public Opinion on Climate Change 6 1.2 The great majority of respondents think climate change is happening. After being asked about their current level of knowledge about climate change, respondents were given a short definition of climate change: "Climate change refers to the idea that the world's average
The role of science in environmental studies and climate change. - Oreskes, “The Scientific Consensus on Climate Change,” Climate Change, chapter 4 Unit 2: A Primer on Climate Change Science and Why It Is So Controversial Tu 2/10 Science 1: Climate Change Basics - Mann and Kump, D
Climate Connections. Climate Change en español. Frequent Questions Energy and the Environment Climate and Energy Resources for State, Local, and Tribal Governments Facility Greenhouse Gas Emissions Data Climate and Transportation Climate and Water EPA Climate Change Research Contact Us. to ask a question, provide feedback, or report a problem .
Breaking the climate-finance doom loop Finance Watch Report June 2020 4 Executive summary Tackling financial instability induced by climate change Urgent action is needed to tackle the climate-finance doom loop, in which fossil fuel finance enables climate change, and climate change threatens financial stability. Action by regulators
