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Climate Change Scenarios and Projections

(prepared by Peter Johnston)

References to chapters, figures and tables are from IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group 1 to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds)] Cambridge University Press, Cambridge, UK and New York, NY, USA, 881pp.

 

 

Introduction

A climate scenario refers to a plausible future climate constructed for investigating the potential consequences of anthropogenic climate change. Climate scenarios account for both human-induced climate change and natural climate variability. A climate scenario can be distinguished from a climate projection, which describes the response of the climate system to a scenario of greenhouse gas and aerosol emissions, as simulated by a climate model. Model outputs in climate projections commonly have to be combined with observed climate data to be usable as inputs to models that estimate future impacts of climate change.

Table 1 presents an example of using climate projections to construct a climate scenario. It also illustrates some other considerations in performing an impact assessment.

Table 1: Example of scenario construction for an impact study: climate change and world food supply.

Aim of the study The study objective was to estimate how global food supply might be affected by climate change up to the year 2060. The change in yield of major crop staples was estimated under different scenarios using crop models for 112 representative sites distributed across the major agricultural regions of the world. The estimated changes in yield were assumed to be applicable to large regions, and used to estimate changes in total production that became inputs to a global trade model. Using assumptions about future population, economic growth, trading conditions and technological progress, the trade model estimated plausible prices of food commodities on the international market given supply as defined by the production estimates. This information was used to estimate the number of people in developing countries who would be at risk from hunger.

Scenario information Scenario information required for the analysis includes:

  • scenarios of carbon dioxide (CO2) concentration
  • climate observations and scenarios of future climate
  • adaptation scenarios (e.g., new crop varieties, adjusted farm management)
  • scenarios of regional population and global trading policy

Scenarios of population and Gross Domestic Product (GDP) were broadly in line with the transient scenario of greenhouse gas emissions, and hence CO2 concentrations. The climate scenarios were based on 2xCO2equilibrium General Circulation Model (GCM) projections from three models, where the radiative forcing of climate was interpreted as the combined concentrations of CO2 and other greenhouse gases equivalent to a doubling of CO2, assumed to occur in about 2060.

Construction of the climate scenario Projections of current (and hence future) regional climate from the GCM simulations were not accurate enough to be used directly as an input to the crop model, so modeled changes in climate were applied as adjustments to the observed climate at a location. Climate change by 2060 was computed as the difference (air temperature) or ratio (precipitation and solar radiation) of monthly mean climate between the GCM (unforced) control and 2xCO2 simulations at GCM grid boxes coinciding with the crop modeling sites. These estimates were used to adjust the observed time series of daily climate for the baseline period (usually 1961 to 1990) at each site. Crop model simulations were conducted for the baseline climate and for each of the three climate scenarios, with and without CO2 enrichment (to estimate the relative contributions of CO2 and climate to crop yield changes), and assuming different levels of adaptation capacity.

 

Difference between climate and climate change scenario

We also distinguish between a climate scenario and a climate change scenario. The latter term sometimes denotes a plausible future climate. However, this term should refer to a representation of the difference between some plausible future climate and the current or control climate (usually as represented in a climate model). A climate change scenario can be viewed as an interim step toward constructing a climate scenario. A climate scenario usually combines the climate change scenario with a description of the current climate as represented by climate observations. It is the contrasting effects of these two climates – one current (the observed “baseline” climate), one future (the climate scenario) that determines the impact of the climate change.

Scenarios are neither predictions nor forecasts of future conditions. Rather they describe alternative plausible futures. The purpose of scenarios is to illuminate uncertainty; they help in determining the possible ramifications along one or more plausible (but indeterminate) paths.

Not all possibly imaginable futures can be considered viable scenarios of future climate. For example, most climate scenarios include the characteristic of increased lower tropospheric temperature (except in some isolated regions and physical circumstances), because most climatologists have very high confidence in that characteristic. Given our present knowledge, a scenario that portrayed global tropospheric cooling for the 21st century would not be viable. What constitutes a viable scenario of future climate evolves along with our understanding of the climate system.

It is worth noting that the development of climate scenarios predates the issue of global warming. In the mid-1970s, for example, when concern emerged regarding the possible global cooling effect of aircraft on the stratosphere, simple incremental scenarios of climate change were formulated to evaluate what the possible effects might be world-wide.

 

Next: Why Do We Need Climate Scenarios?