Climate predictions for the future

Question 1

The model chain

Answer 1

Emission Scenarios
↓
Global Climate Model (GCM)
↓
[Dynamical downscalling] Regional Climate Model (RCM) and [Statistical downscalling] Statistical Model
↓
[Bias correction] Hydrological Model (HM)

Question 2

Emission Scenarios

Answer 2

Scenario:
An outline or model of an expected or supposed sequence of events.

Emission Scenarios
Images of the future, or alternative futures. They are neither predictions nor forecasts. Rather, each scenario is one alternative image of how the future might unfold. (IPCC)

Available scenarios
Currently, there are two different sets of emission scenarios in use: SRES (since 2000) and RCP (since 2014)

Question 3

SRES: The emission scenarios of the IPCC special

report on emission scenarios (Nakicenovic et al., 2000)

Answer 3

 A2: divided world, regional economic growth, permanent increasing population

 A1B: integrated world, rapid economic growth, decline of population from mid 21st
century, balanced use of
energy sources

 B1: global solutions für social, ecological and economic problems, sustainable development, otherwise like A1B,

Question 4

RCP: The Representative Concentration Pathways (IPCC, 2014)

18

Answer 4

 The Representative Concentration Pathways (RCPs) describe four different 21st century pathways of greenhouse gas (GHG) emissions and atmospheric concentrations, air pollutant emissions and land use.

 They include a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0), and one scenario with very high GHG emissions (RCP9.5).

 The numbers represent the radiative forcing in 2100 relative to 1750 in W/m2

 The RCPs cover a wider range than the SRES scenarios, as they also represent scenarios with climate policy.

 In terms of overall forcing, RCP9.5 is broadly comparable to the SRES A2/A1FI scenario, RCP6.0 to B2 and RCP4.5 to B1. For RCP2.6, there is no equivalent scenario in SRES.

Question 5

Climate models:

General Circulation Models (GCM)

Answer 5

 Are based on physical laws and able to simulate the global fully coupled climate system consisting of Atmosphere, Ocean, Land
surface, Ice and snow surfaces and Biosphere (see Fig. 9.1)

 Are the primary tools available for investigating the response of the climate system to various forcings

 Used for simulations of the past climate

 Used for climate predictions on seasonal to decadal time scales

 Used for projections of future climate over coming century/ies

 Atmosphere–Ocean General Circulation Models (AOGCMs) were the ‘standard’ climate models in the 4th assessment report (AR4)

 The Earth System Models (ESMs) are the current state-of-the-art models, they include also various biogeochemical cycles.

Question 6

Climate models:

Regional Circulation Models (RCM)

Answer 6

 Are based on physical laws and describe a regional section of the climate system

 Are driven by initial and boundary conditions of GCMs

 Are mostly used for simulations and predictions of the climate up to the end of the century

 They can be considered as tools for dynamic downscaling of GCM simulations (see also Chap. 9.4.4)

Question 7

Downscaling and bias correction:

Downscaling

Answer 7

 Horizontal resolution of General Circulation Models (GCMs) is too coarse for regional climate impact assessment

 Especially variables like precipitation, temperature, wind speed etc. are needed at a higher resolution

 Calculation of variables at fine scale from coarse scale information (predictors) is called downscaling

Question 8

Downscaling types

Answer 8

Dynamic Downscaling (DD):

Statistical Downscaling (SD):

Statistical-dynamical Downscaling
(SDD):

Question 9

Dynamic Downscaling (DD):

Answer 9

Deterministic modelling of local variables using one or more Regional Climate Models (RCM) nested within a Global Climate Model (GCM)

Question 10

Statistical Downscaling (SD):

Answer 10

Calculation of local variables (predictands) using empirical relations between local observations and large scale atmospheric variables (predictors) provided by GCMs

Question 11

Statistical-dynamical Downscaling

(SDD):

Answer 11

Using both RCMs and statistical relations to calculate local variables from GCMs

Question 12

Validation of climate models:

Answer 12

 In order to evaluate climate models (CM) a comparison of simulations with observations for past periods is needed

 Usually surface climate variables like temperature, precipitation, radiation, humidity are of special interest for hydrological impact studies

 The mismatch between the spatial scales of point observations and grid based simulations of CMs has to be considered; usually RCMs are used for validation

 The simulations of the CMs provide time series which don’t match directly with observations to a specific time but only statistically for a time period

Question 13

Bias correction:

Answer 13

 A systematic error between observations and climate model simulations is called a bias

 The bias correction is trying to remove the systematic error by correcting the simulations

 There are simple methods like linear scaling:

 For applying bias correction it is assumed that the bias is stationary with time and valid also for the future (f)

 There are also more advanced methods like quantile mapping, where the correction is non-linear depending on the distributions of X (see Fig. 9.11)