Climate change and the AR6 IPCC report: what’s new?

The latest IPCC report is out!  It contains the latest scientific update about what has happened, and what could happen with our world’s climate.

The Physical Science report summarizes the knowledge of climate science gathered by 117 scientists from all over the world. If you don’t have time to go through 3,949 pages of science, the Summary for Policymakers is an excellent synthesis - all in 42 pages only! This article is based on the summary report, and will outline some issues evoked by dealing with two broad questions. First, what is the situation now? Second, what could (or will) occur in the future?

Any figure mentioned in this article is sourced directly from the report.

1. The climate situation
   

In 2019, CO2 (carbon dioxide) concentrations reached 410 ppm in the Earth’s atmosphere. This greenhouse gas (GHG), along with other GHGs, such as CH4 (methane), tends to amplify the greenhouse effect. In short, incoming solar radiation ‘rebounds’ on the ground and is redirected into the atmosphere. However, GHGs act as reflectors of radiation, which redirect it to the ground (etc.), defining what is called the greenhouse effect.

Without the greenhouse effect, we would all be dead, as the Earth would be uninhabitable. However, too much of it traps excessive energy within our atmosphere and causes global warming. As a result, the report informs us that each of the last four decades has been warmer than the previous, with 1980-1990 warmer than any of the decades since 1850:

The global surface temperature was 1.09 degrees Celsius warmer than 1850-1900. Although temperature and greenhouse gases concentrations in the atmosphere vary naturally (and define glacial and interglacial periods), CO2 and CH4 increases since 1750 are unprecedented over the last 800,000 years.

Besides, oceans have warmed and become more acidic globally. The current surface open ocean pH (measure of acidity) is unprecedented in the last 2 million years. Because the oceans warm, they expand (this is called ‘thermal expansion’). That, melting glaciers and ice sheets explain most of the sea level rise taking place. The global sea level has increased by about 20 cm between 1900 and 2015. Sea level rise has accelerated in the last decades, culminating at 3.7 mm/year between 2006 and 2018.

The current situation is already a threat to many livelihoods across the globe. However, the biggest challenge remains ahead of us, as what could happen may be the most worrying.

2. The future climate

Before we start discussing the future of climate, let's talk about acronyms.

GCMs and SSPs

To forecast the future climate, scientists have developed numerous models that simulate the behavior of climatic variables in the entire world. Such models, called Global Climate Models (GCMs), are regularly updated as mankind’s knowledge improves. The latest IPCC report (AR6) is based on a new generation of models, gathered in the 6th generation of the Coupled Model Intercomparison Project (CMIP6).

Some aspects of the future climate will depend on choices that have not yet been made. Thus, scientists designed scenarios that can be used to describe our choices. Will we pollute more, or less? This will have consequences for the Earth’s future, which can be described by GCMs. Those scenarios are the Shared Socioeconomic Pathways (SSPs). The most optimistic SSPs consider that the quantity of CO2 will peak and then decline within the 21st century. The most pessimistic are based on the scenario where the world keeps increasing its GHG emissions up until (almost) 2100.

Temperature in the future

Whatever happens, global temperature will continue to increase until at least 2050 due to our past GHG emissions. Intermediate emissions scenarios (SSP2-4.5) lead to a bit less than a 3 degree Celsius increase in global surface temperature by 2100, while pessimistic scenarios go over 4 degrees:

Thus, unless deep reductions in GHGs occur in the coming decades, we will fail to limit global warming under 2 degrees, an objective defined in the Paris Agreement.

Future hot extremes, heavy precipitation, and droughts

The intensity and frequency of hot extremes, heavy precipitation events, and droughts will increase with each additional half degree of global warming. You will find below three examples of the consequences global warming has on such events, based on an intermediate scenario involving a 2 degree global warming, using information found on page 24 of the report.

Hot temperature extremes over land

A hot temperature extreme that would have occurred once in 50 years before 1900 would occur 13.9 times with a global warming of about 2 degrees (that is, in the decades preceding 2080 in an intermediate scenario).

Heavy precipitation over land

A heavy precipitation event occurring once every 10 years would occur 1.7 times under a 2 degrees warming scenario.

Drought in drying regions (such as the Western United States).

A heatwave occurring once every 50 years before 1900 occurs 4.8 times at present and up to 39.2 times in pessimistic scenarios.

Future sea level rise

As the ocean becomes warmer, glaciers and ice sheets melt and the sea level rises. This mechanism will be amplified in the coming decades, centuries and millennia. Relative to 1995-2014, the lowest GHG emissions scenario considers that the sea level will rise by 28 to 55 cm by 2100, and up to 1.88 m under the very high scenario. This will pose challenges to numerous areas, such as the Keys in Florida, the Maldives, Seychelles or the Torres Strait Islands. However, many locations along the U.S. East Coast are already at risk of coastal flooding due to sea level rise.

By the way, if you’re interested in your flood risk, our coastal flood risk assessment is available everywhere along the U.S. Coastline and takes into account the elevation in your block, along with your expected sea level rise by 2050 in a pessimistic scenario:

Screenshot: the Augurisk Coastal Flood assessment for a random location in New Orleans, Louisiana.

Whatever happens in the coming years and decades, the global sea level will continue to rise in the centuries and millennia to follow, and the climate will evolve. However, the amplitude of this phenomenon will be determined by our capacity to limit global warming.

In order to limit global warming to 1.5 degrees Celsius with an 83% chance, 300 GtCO2 should be emitted at most, worldwide, from early 2020 on.  That figure is not a yearly amount: we are speaking here about the total amount of CO2 we can afford to add to the atmosphere to stay within the 1.5 degrees threshold. After that, we must reduce our emissions to 0. Yes, 0, which means that any gram of CO2 will have to be compensated somehow by then. In 2020 alone, the International Energy Agency estimates that the worldwide energy sector already emitted 30 GTCo2.  Are you ready to act?

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