Characterizing half‐a‐degree difference: a review of methods for identifying regional climate responses to global warming targets

Rachel James,Declan Conway,Richard Washington,Carl‐Friedrich Schleussner,Joeri Rogelj

doi:10.1002/wcc.457

Abstract

The Paris Agreement long‐term global temperature goal refers to two global warming levels: well below 2°C and 1.5°C above preindustrial. Regional climate signals at specific global warming levels, and especially the differences between 1.5°C and 2°C, are not well constrained, however. In particular, methodological challenges related to the assessment of such differences have received limited attention. This article reviews alternative approaches for identifying regional climate signals associated with global temperature limits, and evaluates the extent to which they constitute a sound basis for impacts analysis. Four methods are outlined, including comparing data from different greenhouse gas scenarios, sub‐selecting climate models based on global temperature response, pattern scaling, and extracting anomalies at the time of each global temperature increment. These methods have rarely been applied to compare 2°C with 1.5°C, but some demonstrate potential avenues for useful research. Nevertheless, there are methodological challenges associated with the use of existing climate model experiments, which are generally designed to model responses to different levels of greenhouse gas forcing, rather than to model climate responses to a specific level of forcing that targets a given level of global temperature change. Novel approaches may be required to address policy questions, in particular: to differentiate between half degree warming increments while accounting for different sources of uncertainty; to examine mechanisms of regional climate change including the potential for nonlinear responses; and to explore the relevance of time‐lagged processes in the climate system and declining emissions, and the resulting sensitivity to alternative mitigation pathways. WIREs Clim Change 2017, 8:e457. doi: 10.1002/wcc.457This article is categorized under: Assessing Impacts of Climate Change > Scenario Development and Application

Highlights

The Paris Agreement under the United Nations Framework Convention on Climate Change (UNFCCC)[1] aims to hold the increase in global mean surface air temperature to well below 2C relative to preindustrial levels and to pursue efforts to limit it to 1.5C (Box 1)
In this article we focus on atmospheric responses, but for some geophysical impacts, for example, glacial retreat, changes in ice sheets, and sea-level rise, adjustments in response to global temperature could take decades or centuries.[7,98,99]
This article has reviewed studies which identify regional climate signals at specific ΔTg increments, to assess the extent to which their methods provide useful information to distinguish at regional scale between 1.5C and 2C

Summary

INTRODUCTION

The Paris Agreement under the United Nations Framework Convention on Climate Change (UNFCCC)[1] aims to hold the increase in global mean surface air temperature to well below 2C relative to preindustrial levels and to pursue efforts to limit it to 1.5C (Box 1). Concentrations, or radiative forcing scenario, different climate models generate different global temperature responses due to variation in climate sensitivity[47] and aerosol forcing.[48,49] a time slice taken from one scenario, here SRES A2, is associated with a range of ΔTg anomalies, making it difficult to infer implications for regional climate at any specific ΔTg increment, or to compare ΔTg increments. Some studies have used RCP2.6 to represent 1.5C or 2C relative to the preindustrial;[55,56] in some cases comparing it to TABLE 1 | Summary of Advantages and Disadvantages of each of the Four Methodologies

Scenario Approach

SubSelecting Models

Pattern Scaling

Committed

Findings

CONCLUSION