Report

Dans cette contribution au cinquième Rapport d’évaluation du GIEC (RE5), le Groupe de travail I (GTI) examine de nouveaux éléments concernant le changement climatique sur la base de nombreuses analyses scientifiques indépendantes d’observations du système climatique, d’archives paléoclimatiques, d’études théoriques des processus climatiques et de simulations à l’aide de modèles climatiques. Il s’appuie sur sa contribution au quatrième Rapport d’évaluation du GIEC (RE4) et incorpore de nouveaux résultats de recherche obtenus depuis. Composante du cinquième cycle d’évaluation, le rapport spécial intitulé Gestion des risques de catastrophes et de phénomènes extrêmes pour les besoins de l’adaptation au changement climatique (SREX) représente un socle d’informations important sur l’évolution des extrêmes météorologiques et climatiques. Le présent Résumé à l’intention des décideurs suit la structure du rapport du Groupe de travail I. Ce texte s’accompagne d’une mise en exergue des principales conclusions qui, rassemblées, fournissent un résumé concis. L’introduction des principales sections se présente sous la forme d’un bref paragraphe en italique, qui souligne les bases méthodologiques de l’évaluation.

Climate Change 2013: The Physical Science Basis is the contribution of Working Group I (WGI) to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC). This comprehensive assessment of the physical aspects of climate change puts a focus on those elements that are relevant to understand past, document current and project future climate change. The assessment builds on the IPCC Fourth Assessment Report (AR4)1 and the recent Special Report on Managing the Risk of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX)2 and is presented in 14 chapters and 3 annexes. The chapters cover direct and proxy observations of changes in all components of the climate system; assess the current knowledge of various processes within, and interactions among, climate system components, which determine the sensitivity and response of the system to changes in forcing; and quantify the link between the changes in atmospheric constituents, and hence radiative forcing (RF)3, and the consequent detection and attribution of climate change. Projections of changes in all climate system components are based on model simulations forced by a new set of scenarios. The Report also provides a comprehensive assessment of past and future sea level change in a dedicated chapter. Regional climate change information is presented in the form of an Atlas of Global and Regional Climate Projections (Annex I). This is complemented by Annex II: Climate System Scenario Tables and Annex III: Glossary.

The Working Group I contribution to the IPCC’s Fifth Assessment Report (AR5) considers new evidence of climate change based on many independent scientific analyses from observations of the climate system, paleoclimate archives, theoretical studies of climate processes and simulations using climate models. It builds upon the Working Group I contribution to the IPCC’s Fourth Assessment Report (AR4), and incorporates subsequent new findings of research. As a component of the fifth assessment cycle, the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) is an important basis for information on changing weather and climate extremes.

This Summary for Policymakers (SPM) follows the structure of the Working Group I report. The narrative is supported by a series of overarching highlighted conclusions which, taken together, provide a concise summary. Main sections are introduced with a brief paragraph in italics which outlines the methodological basis of the assessment.

Explore terms that are used in the report to better understand key concepts and the meaning behind how terms are used.

The interactions of climate change and climate responses with sustainable development including sustainable development impacts at 1.5C and 2C, the synergies and tradeoffs of mitigation and adaptation with the Sustainable Development Goals/SDGs, and the possibilities for sustainable and equitable low carbon, climate resilient development pathways.

This chapter takes sustainable development as the starting point and focus for analysis. It considers the broad and multifaceted bi-directional interplay between sustainable development, including its focus on eradicating poverty and reducing inequality in their multidimensional aspects, and climate actions in a 1.5°C warmer world. These fundamental connections are embedded in the Sustainable Development Goals (SDGs). The chapter also examines synergies and trade-offs of adaptation and mitigation options with sustainable development and the SDGs and offers insights into possible pathways, especially climate-resilient development pathways towards a 1.5°C warmer world.

The global response to warming of 1.5oC comprises transitions in land and ecosystem, energy, urban and infrastructure, and industrial systems. The feasibility of mitigation and adaptation options, and the enabling conditions for strengthening and implementing the systemic changes, are assessed in this chapter.

Limiting warming to 1.5°C above pre-industrial levels would require transformative systemic change, integrated with sustainable development. Such change would require the upscaling and acceleration of the implementation of arreaching, multilevel and cross-sectoral climate mitigation and addressing barriers. Such systemic change would need to be linked to complementary adaptation actions, including transformational adaptation, especially for pathways that temporarily overshoot 1.5°C (medium evidence, high agreement). Current national pledges on mitigation and adaptation are not enough to stay below the Paris Agreement temperature limits and achieve its adaptation goals. While transitions in energy efficiency, carbon intensity of fuels, electrification and land-use change are underway in various countries, limiting warming to 1.5°C will require a greater scale and pace of change to transform energy, land, urban and industrial systems globally.

Why is it necessary and even vital to maintain the global temperature increase below 1.5°C versus higher levels? Adaptation will be less difficult. Our world will suffer fewer negative impacts on intensity and frequency of extreme events, on resources, ecosystems, biodiversity, food security, cities, tourism and carbon removal.

Showing how emissions can be brought to zero by mid-century stay within the small remaining carbon budget for limiting global warming to 1.5°C.

This chapter assesses mitigation pathways consistent with limiting warming to 1.5°C above pre-industrial levels. In doing so, it explores the following key questions: What role do CO2 and non-CO2 emissions play? To what extent do 1.5°C pathways involve overshooting and returning below 1.5°C during the 21st century? What are the implications for transitions in energy, land use and sustainable development? How do policy frameworks affect the ability to limit warming to 1.5°C? What are the associated knowledge gaps?

This chapter frames the context, knowledge-base and assessment approaches used to understand the impacts of 1.5°C global warming above pre-industrial levels and related global greenhouse gas emission pathways, building on the IPCC Fifth Assessment Report (AR5), in the context of strengthening the global response to the threat of climate change, sustainable development and efforts to eradicate poverty.

An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty