Climate sensitivity

Climate sensitivity is a parameter that sets the global warming of the atmosphere in relation to a radiation unit by the action of greenhouse gases. They can be in degrees Celsius per watt per square meter (° C / (W / m )) = ° C · m / W. ). Common, however, is the indication of global warming with a doubling of the concentration of carbon dioxide in Earth's atmosphere. This means that the average temperature of the earth by this amount increases when increasing the concentration of CO2 from pre-industrial 280 ppm to 560 ppm then.

The exact knowledge of climate sensitivity is for the future evolution of the climate of fundamental importance, because it allows the resultant of a certain concentration of greenhouse gases warming can be calculated. The value of climate sensitivity depends on the initial climate state and can potentially be accurately determined from climate proxies.

In addition to carbon dioxide even more gases to the greenhouse effect contribute, so that can also be determined for each with its own climate sensitivities. For simplicity, their contribution is usually calculated by means of the so-called CO2 equivalents.

Background

When considering only the radiation measured in the laboratory effect of CO2 results in a climate sensitivity of 1.2 ° C. For climate sensitivity but also bears the sum of all feedbacks in Erdklimasystem at, such as the response of sea level ( feedback in the cryosphere ) depending on the planetary energy balance. A distinction is made between fast and slow feedback. Water vapor, ice albedo and aerosol feedback and clouds are considered fast feedback effects. The ice sheets, changes in vegetation and the concentration of the greenhouse gas CO2 are considered slow feedback effects. The response of ice sheets and CO2 in the air increase the climate sensitivity by an amount that depends on the observation period. In this current climate models overestimate the hysteresis of the ice sheets. This means that the response of ice sheets to climate change not as strongly depends on whether the earth is in a state of a heating or a cooling than is commonly assumed. The air is therefore a sensitivity dynamic factor that depends on the respective air condition. Models and the geological history show that the climate sensitivity with the increase in radiative forcing, that is, with increasing global temperature also rises.

Range of research results

Since the discovery of the warming effect of carbon dioxide many different values ​​for the climate sensitivity have been published. Svante Arrhenius was based on a climate sensitivity of 5.5 ° C in 1896. Guy Stewart Callendar came in 1938 to 2 ° C. The range of published values ​​ranging from 0.1 ° C ( Sellers, 1973) to 9.6 ° C ( Fritz Moller, 1963). The National Academy of Sciences warned as the world's first great science organization of a global warming and gave in 1979 in the Charney Report, the climate sensitivity of 3 ° C (± 1.5 ° C), which even today is considered the standard. A study from 2006 was based on the combined estimates based on the Bayes' theorem to a 95% probability of 1.5 ° C and 4.5 ° C climate sensitivity.

The Intergovernmental Panel on Climate Change (IPCC ) stated in its Fourth Assessment Report published in 2007 values ​​between 2 and 4,5 ° C as " likely" to. The best average estimate lies at 3 ° C and a sensitivity of less than 1.5 ° C was "very unlikely". In the fifth progress report, which was published in 2013, this likely range was changed to a range between 1.5 and 4.5 ° C. This specification is identical to the Third Assessment Report of 2001.

Determination of climate sensitivity

Depending on the method of determination results in a different value for the climate sensitivity. In 2005, could be measured, that the earth receives 0.85 W / m² more energy than it emits into space. In an 8 -year series of measurements an increase in long -wave atmospheric radiation counter was occupied by measurement of the anthropogenic greenhouse effect werden.Der far the largest portion of the measured additional radiative forcing was expected due to the positive feedback by clouds and water vapor. However, to calculate the climate sensitivity measurements this kind are not suitable because in this case many of the acting in the climate system feedbacks are ignored. By different methods an attempt is made to reduce uncertainties in the determination of the sensitivity of air:

Paleoclimate methods

Paleoclimatological several studies have attempted to determine the air sensitivity of the last several million years. A 2007 published in the journal Nature paleoclimate study examines the climate sensitivity over the last 420 million years. The average global temperature and the concentration of greenhouse gases were subjected in this very long period strong fluctuations and the radiation output of the sun increased in this period by about 4 %, which is a good prerequisite for basing estimate of climate sensitivity with low margin of error. Unfortunately, the climate archives of the ice cores are hardly enough more than one million years into the past and the arrangement of land masses was during this time have undergone significant changes, so that there is great uncertainty about many climate- determining parameter. Thus, results from these studies, a relatively large uncertainty, which brings 1.5 ° C lower than 6.2 ° C and the upper limit and 2.8 ° C as the best estimate.

In a published work in 2012, the results of several studies were collectively evaluated who had the last 65 million years in focus. This yielded a 95% chance of a value for the climate sensitivity, which is in the range between 2.2 ° C and 4.8 ° C.

Regression analyzes

With good knowledge of all climate- determining factors can try to isolate the climate sensitivity using a regression analysis. For this, the ice age cycles of the past year, hundreds of thousands are investigated. During this time, the concentration of CO2 and temperatures fluctuated widely, while others climatological effective parameters are not very different from the present situation. Ice cores, which were obtained since the 1990s in various places on Earth, shed light on the prevailing concentrations of greenhouse gases, aerosols and precipitation, as well as the temperature profiles of the last approximately 1 million years.

Climate models

The current and future climate can only be properly simulated, even if the climate sensitivity was determined correctly. Therefore, climate models are tested if they can check the current, but also the climate during the ice ages simulate correctly. In the context of such simulations over 1000 models are expected, with input parameters are varied within their assumed fault width. Models that reflect the temperature variation in the period is incorrect ( > 90%) are sorted out. With this method, 3 ° C and 3.4 ° C were found as the most probable values ​​for the climate sensitivity. Considering the temperature changes during the last ice age, you could based on ice cores, a temperature change of 5 ° C with one of the Milankovitch cycles and feedbacks (albedo, vegetation, aerosols, CO2) resulting changes in radiative forcing of 7.1 W / m² link. The resulting calculated climate sensitivity is 5/7, 1 = 0.7 K · W-1 · m2. You can use these empirically determined climate sensitivity for the calculation of a radiative forcing of 4 W / m² resulting temperature rise, which corresponds to a doubling of atmospheric CO2 concentration compared to pre-industrial levels. As a result, shows an increase of 3 ° C.

Earth-system climate sensitivity

The CO2 concentration in 2007 of approximately 380 ppm, together with the other greenhouse gases to a radiative forcing of 2.6 W / m². This radiative forcing leads to a global warming of 2 ° C, when calculating with the most probable value for the climate sensitivity of 3 ° C. However, the warming reached after decades to centuries its maximum, because the climate because of the high heat capacity of the water masses of the oceans to be quite slow. Even if greenhouse gas concentrations would have been frozen at the 2000 level, the climate would therefore still global warming by the end of the century by 0.6 ° C. And so the was by 2007 global warming of 0.7 ° C only half to two-thirds of the then existing CO2 concentration expected value. The melting of large quantities of ice - as they exist eg in Greenland and in the Antarctic - required many centuries and the heating runs, inter alia, because of the ice - albedo feedback, even at a complete stop emission of these periods continue. In addition, a climate change leads to growth changes. Forest absorbs considerably more incident beams as, for example, the relatively bright surface of the tundra.

About half of today emitted into the atmosphere the carbon dioxide enters in the form of carbonic acid in the oceans. Since the solubility of CO2 in water is temperature-dependent, a warming of the oceans will reduce their storage capacity for this greenhouse gas; Model studies indicate that the biosphere is a CO2 sink into a CO2 source from about the end of the 21st century. From the analysis of ice cores we know that climate warming could increase with a certain time lag, the concentration of greenhouse gases, leading to increased warming continues. Even a precise knowledge of climate sensitivity and greenhouse gas emissions therefore does not make an assessment of future climatic development. In 2007 appeared the IPCC climate report this expansion in the A2 scenario with an additional degree Celsius temperature rise by the year 2100 has been considered.

The Earth system climate sensitivity ( engl. Earth System Sensitivity, ESS), also includes this response of the climate. With doubling of CO2 concentration, the Earth system climate sensitivity is about 4-6 ° C when the ice caps and the albedo - vegetation feedback into account and is even higher when considering the greenhouse gas feedbacks. Hansen et al. 2013 calculated with the Earth system climate sensitivity has a value of 3-4 ° C based on a 550 ppm CO2 scenario. Previdi et al. 2013 calculated based on the Earth system climate sensitivity about 4-6 ° C without consideration of greenhouse gas feedbacks.

Implications

The atmospheric concentration of carbon dioxide will not fall at total emission stop in a natural way, even in periods of centuries. To stop the anthropogenic global warming, hence a large reduction of greenhouse gas emissions is not enough, as a published study showed the end of 2008: this, the immediate and complete cessation of emissions of greenhouse gases would be necessary.

Hansen et al. 2013 warns that large parts of the planet could become uninhabitable. The burning of all fossil fuels would lead to an atmospheric CO2 concentration of approximately 1500 ppm, heat the air over the continents by an average of 20 ° C and the poles to 30 ° C. Feedbacks in the climate system might lead to longer periods of time to a similar super greenhouse as on Venus, if greenhouse gas emissions are not limited and partially reversed.

In the atmosphere of Venus hydrogen is contained only in low concentrations; therefore it is believed that almost everything has escaped into space. In this process, preferably escapes the light hydrogen isotope, deuterium remains. The deuterium content in the atmosphere of Venus is a factor of 100 higher than would normally be expected. This is seen as evidence that Venus was once like Earth today had vast oceans, which are eventually vaporized in its history by a self-reinforcing, unchecked global warming. A complete evaporation of the terrestrial oceans through the man-made warming, however, is virtually eliminated.