Modelling the climate system and climate change PRECIS Workshop Tanzania Meteorological Agency, 29 th June – 3 rd July 2015

The Climate System The goal of this session is a brief introduction to: the climate system drivers of climate modelling the climate system climate variability predicting future changes and impacts

Session contents 1.Introduction 2.The Greenhouse Effect 3.Climate Variability 4.Climate of the 20th Century 5.Climate Models 6.Predicting Climate Change & Impacts

1. Introduction

What is the Climate System? The complicated system consisting of various components, including the dynamics and composition of the atmosphere, the ocean, the ice and snow cover, the land surface and its features, the many mutual interactions (ie feedbacks) between them, and the large variety of physical, chemical and biological processes taking place in and among these components. Climate refers to the state of the climate system as a whole, including a statistical description of its variations.

Components of the climate system

Planetary energy balance A planetary object intercepts a circle (of radius R) of incoming solar energy S as S  R 2 A (for Albedo) of which is reflected back into space. Energy absorbed is balanced by radiation to space. Hence: S  R 2 (1-A) = 4  R 2  T 4 4 therefore T = [ S(1-A)/4  ] 

Planetary energy balance For the Moon, S = 1365 Wm -2 A = 0.1 results in.... T = 272 K

Planetary energy balance For the Earth, S = 1365 Wm A = 0.3 results in.... T = 255 K (-18 0 C) In fact, the mean surface temperature is T = 287 K (14 0 C)

2. The Greenhouse Effect

The Greenhouse Effect Visible energy from the sun passes through the glass and heats the ground Infra-red heat energy from the ground is partly reflected by the glass, and some is trapped inside the greenhouse

The Greenhouse Effect Some solar radiation is reflected by the earth’s surface and the atmosphere EARTH Most solar radiation is absorbed by the surface, which warms Some of the infrared radiation is absorbed and re-emitted by the greenhouse gases. The effect of this is to warm the surface and the lower atmosphere Infrared radiation is emitted from the Earth’s surface

3. Climate Variability

Changes in certain components of the climate system perturb the radiative energy budget of system, i.e. provide a radiative forcing. Examples include: the concentration of radiatively active species solar irradiance changes affecting radiation absorbed by the surface Human induced perturbations include composition of the atmospheric gases increases in atmospheric aerosols land-use change (agriculture, deforestation, reforestation, afforestation, urbanisation, …) The concept of radiative forcing

External forcings: solar radiation volcanic eruptions = aerosol emission Internal climate variability: ENSO NAO and other leading modes of variability Natural variability of climate

Impact of the Mt. Pinatubo eruption

Perturbations of the atmospheric composition : The enhanced greenhouse effect Aerosol direct effect (scattering of incoming solar radiation) Aerosol indirect effect (affecting the radiative properties of clouds) Land-use change (agriculture, deforestation, reforestation, afforestation, urbanisation, traffic, …) Human-induced climate variations

The Enhanced Greenhouse Effect

Carbon dioxide rising CO 2 from Ice core records

Recent CO 2 observations from Hawaii

Indicators of the human influence IPCC AR4 WG1

Human Perturbation of the global carbon cycle Large amounts naturally in/out of atmosphere - but in long term these balance IPCC AR4 WG1

Radiative Forcings IPCC AR4 WG1

The effect of aerosol

The response of the climate system to these forcing agents is complicated by: feedbacks the non-linearity of many processes different response times of the different components to a given perturbation The only means available to calculate the response is by using numerical models of the climate system. How do we quantify the response of the climate?

What are the processes which feedback on the climate? Source: Intergovernmental Panel on Climate Change (IPCC), WG1-AR3, Ch 1. Processes which represent feedbacks (Things not in a red box are mostly part of  Q or a mixture.)

4. The Climate of the 20th Century

Variations of the Earth’s surface temperature 1850 to 2005

SPM 1b Variations of the Earth’s surface temperature for the past 1000 years

Natural only Natural and anthropogenic Past climate change predicted by climate models

5. Climate Models

Climate modelling starts with basic energy balance The basics of the greenhouse effect have been understood for over a century! A simple energy balance model can be written down on a page of equations...

Earth-system processes in today’s climate models

In principle, climate models are a large set of equations describing the physical and chemical processes occurring in the atmosphere, ocean, land, ice... etc

Example equations for the motion of the Atmosphere

Equations solved in “grid-cells” of a massive global grid

Hadley Centre Global Climate Model FORTRAN program code

In practice these equations have to be solved trillions of times in a climate-run. The equations are “coded-up” into in a large computer program (climate model) This solves the equations numerically

Development of Climate models

6. Predicting Climate Change

Predicting Climate Change

Climate Model Projections

Global average 5.5 ºCGlobal average 1.9 ºC Strong mitigation scenario No mitigation scenario Results based on Hadley Centre climate model HadGEM2-ES (contribution to IPCC 2013) Precipitation changes 2071 to 2100 Relative to 1990

Strong mitigation scenario No mitigation scenario Results based on Hadley Centre climate model HadGEM2-ES (contribution to IPCC 2013)

Global average sea level rise by 2100 Thermal expansion of water, is the strongest contributor to sea level rise in these projections. The contribution from melting of polar ice sheets is likely to be within the range 0.03 – 0.20m by The Greenland ice sheet stores the equivalent of 6m sea level rise RCP8.5 (no mitigation) RCP2.6 (strong mitigation) Figure from IPCC 2013

Ocean warming & acidification

Questions & answers