Klimaänderung. Robert Sausen Deutsches Zentrum für Luft- und Raumfahrt Institut für Physik der Atmosphäre Oberpfaffenhofen

Transcription

1 Klimaänderung Robert Sausen Deutsches Zentrum für Luft- und Raumfahrt Institut für Physik der Atmosphäre Oberpfaffenhofen Vorlesung WS 2017/18 LMU München

2 8. Anthropogener und natürlicher Strahlungsantrieb

3 Contents of IPCC 2013 Working Group I: the Physical Science Basis

4 Contents of IPCC 2013 Working Group I: the Physical Science Basis

5 Statements in the Executive Summary It is unequivocal that anthropogenic increases in the well-mixed greenhouse gases (WMGHGs) have substantially enhanced the greenhouse effect, and the resulting forcing continues to increase. Aerosols partially offset the forcing of the WMGHGs and dominate the uncertainty associated with the total anthropogenic driving of climate change

6 Radiative budget of the Earth: without atmosphere S0 1 A 4 equilibrium = solar irradiation 4 S T S terrestial radition equilibrium of in-coming and out-going radiation S 0 = solar constant (1368 W/m 2 ) A = albedo (0.3) = emissivity of the soil (0.95) = Boltzmann constant T S = surface temperature soil T S S 1 A C 4 1

7 Radiative budget of the Earth: with atmosphere S0 1 A 4 soil atmosphere 4 T 1 A S S T 4 S S 4 A T A 4 A T A equilibrium of in-coming and outgoing radiation S 0 = solar constant (1368 W/m 2 ) A = albedo (0.3) = emissivity of the soil (0.95) = Boltzmann constant T S = surface temperature T A = atmospheric temperature 7 T S 1 4 S 1 A S 1 A C TA C S A 15 ; A

8 Statements in the Executive Summary It is unequivocal that anthropogenic increases in the well-mixed greenhouse gases (WMGHGs) have substantially enhanced the greenhouse effect, and the resulting forcing continues to increase. Aerosols partially offset the forcing of the WMGHGs and dominate the uncertainty associated with the total anthropogenic driving of climate change. As in previous IPCC assessments, AR5 uses the radiative forcing (RF) concept, but it also introduces effective radiative forcing (ERF)

9 What is "radiative forcing"? (simplified) equilibrium RF = 0 perturbed situation RF > 0 T atmosphere atmosphere soil / ocean soil / ocean

10

11 Definition of Radiative Forcing (RF) RF is defined, as it was in AR4, as the change in net downward radiative flux at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, while holding surface and tropospheric temperatures and state variables such as water vapor and cloud cover fixed at the unperturbed values. T = RF

12 Cartoon comparing different definitions of radiative forcing

13 Definition of Effective Radiative Forcing (ERF) ERF is the change in net TOA downward radiative flux after allowing for atmospheric temperatures, water vapour and clouds to adjust, but with surface temperature or a portion of surface conditions unchanged. Although there are multiple methods to calculate ERF, we take ERF to mean the method in which sea surface temperatures and sea ice cover are fixed at climatological values unless otherwise specified. Land surface properties (temperature, snow and ice cover and vegetation) are allowed to adjust in this method. Hence ERF includes both the effects of the forcing agent itself and the rapid adjustments to that agent (as does RF, though stratospheric temperature is the only adjustment for the latter). In the case of aerosols, the rapid adjustments of clouds encompass effects that have been referred to as indirect or semi-direct forcings, with some of these same cloud responses also taking place for other forcing agents

14 Cartoon comparing different definitions of radiative forcing

15 Definition of Effective Radiative Forcing (ERF) ERF is the change in net TOA downward radiative flux after allowing for atmospheric temperatures, water vapour and clouds to adjust, but with surface temperature or a portion of surface conditions unchanged. Although there are multiple methods to calculate ERF, we take ERF to mean the method in which sea surface temperatures and sea ice cover are fixed at climatological values unless otherwise specified. Land surface properties (temperature, snow and ice cover and vegetation) are allowed to adjust in this method. Hence ERF includes both the effects of the forcing agent itself and the rapid adjustments to that agent (as does RF, though stratospheric temperature is the only adjustment for the latter). In the case of aerosols, the rapid adjustments of clouds encompass effects that have been referred to as indirect or semi-direct forcings, with some of these same cloud responses also taking place for other forcing agents. Calculation of ERF requires longer simulations with more complex models than calculation of RF, but the inclusion of the additional rapid adjustments makes ERF a better indicator of the eventual global mean temperature response, especially for aerosols. When forcing is attributed to emissions or used for calculation of emission metrics, additional responses including atmospheric chemistry and the carbon cycle are also included in both RF and ERF. The general term forcing is used to refer to both RF and ERF

16 The basis for the confidence level IPCC 2013, Chap

17 Likelihood terms IPCC 2013, Chap. 1

18 Statements in the Executive Summary Industrial-Era Anthropogenic Forcing (1) The total anthropogenic ERF over the Industrial Era is 2.3 (1.1 to 3.3) W m 2. It is certain that the total anthropogenic ERF is positive. Total anthropogenic ERF has increased more rapidly since 1970 than during prior decades. The total anthropogenic ERF estimate for 2011 is 43% higher compared to the AR4 RF estimate for the year 2005 owing to reductions in estimated forcing due to aerosols but also to continued growth in greenhouse gas RF. Due to increased concentrations, RF from WMGHGs has increased by 0.20 (0.18 to 0.22) W m 2 (8%) since the AR4 estimate for the year The net forcing by WMGHGs other than CO 2 shows a small increase since the AR4 estimate for the year Ozone and stratospheric water vapour contribute substantially to RF. The magnitude of the aerosol forcing is reduced relative to AR4. IPCC 2013, Chap

19 Time evolution of global tropospheric ozone burden

20 Radiative forcing (RF) from different greenhouse gases

21 Time evolution of the radiative forcing from tropospheric and stratospheric ozone

22 Time evolution of RF due to aerosol radiation interaction and BC on snow and ice

23 Statements in the Executive Summary Industrial-Era Anthropogenic Forcing (2) There is robust evidence that anthropogenic land use change has increased the land surface albedo, which leads to an RF of 0.15 ± 0.10 W m 2. Land use change causes additional modifications that are not radiative, but impact the surface temperature, in particular through the hydrologic cycle. These are more uncertain and they are difficult to quantify, but tend to offset the impact of albedo changes. As a consequence, there is low agreement on the sign of the net change in global mean temperature as a result of land use change. IPCC 2013, Chap

24 Change in top of the atmosphere (TOA) shortwave (SW) flux (W m 2 ) following the change in albedo

25 Statements in the Executive Summary Industrial-Era Anthropogenic Forcing (2) There is robust evidence that anthropogenic land use change has increased the land surface albedo, which leads to an RF of 0.15 ± 0.10 W m 2. Land use change causes additional modifications that are not radiative, but impact the surface temperature, in particular through the hydrologic cycle. These are more uncertain and they are difficult to quantify, but tend to offset the impact of albedo changes. As a consequence, there is low agreement on the sign of the net change in global mean temperature as a result of land use change. Attributing forcing to emissions provides a more direct link from human activities to forcing. Forcing agents such as aerosols, ozone and land albedo changes are highly heterogeneous spatially and temporally. IPCC 2013, Chap