The science behind COP26: evidence for climate action

With the United Nations (UN) Climate Change Conference at its 26th edition in Glasgow, UK, head of 197 states are gathering to turn the spotlight on the most challenging environmental problem humanity is facing: climate change. Negotiators representing the parties of UN Framework Convention for Climate Change (UNFCCC) are working on finalizing the pending controversial aspects of the Paris Agreement and accelerating efforts to limit global warming to well below 2°C, above the pre-industrial temperature level, preferably stabilizing the temperature at 1.5°C (above the pre-industrial temperature level) by the end of the current century.

The list of physical evidence of the current global warming is long and the impacts on economies and societies are devastating.  Many UN specialized agencies, including the World Meteorological Organization (WMO), publish annual reports on the most recent climate science to remind climate negotiators and global leaders what is at stake.

The current physical state of climate and the state-of-the-art assessment of the climate futures

Despite the global COVID-19 pandemic resulted in a reduction of 5.4% of human-caused carbon dioxide (CO2) in the atmosphere, the average concentration of CO2 in the first half of 2021 recorded its highest level in the past 800,000 years. CO2, methane (CH4) and nitrous oxide (N2O) are the main greenhouse gases (GHGs) that are responsible for trapping the infrared radiation near the Earth’s surface that, in turn, warm the planet. Observed increases in the well-mixed GHG concentrations in the atmosphere since 1750 are caused by human activities like burning fossil fuels, land use and other activities [1].

The global average mean temperature for 2017-2021 is the warmest of any equivalent period on record. The observed warming from 1850-1900 is about 1.15°C and the best estimate of total human-caused warming is 1.07°C.  We know that the temperature has changed throughout the Earth’s history by observing a rich variety of geological evidence, however, the scale of the recent warming is unprecedented over many centuries to many thousands of years [2]. Four main attributions are identified:

• The warming is almost everywhere: the pattern is more uniform than the past decadal or centennial temperature fluctuations

• The warming is rapid: the rate of warming during the past 50 years has exceeded the rate of any other 50-year period over at least the last 2000 years

• The recent warming reversed a long-term cooling trend, that started 6500 years ago

• The temperature of the past decade exceeds those of the most recent multi-century warm period, around 6500 years ago. Prior to that, the next most recent warm period was about 125 thousands years ago.

There is more evidence that the human-induced warming is affecting many weather and climate extremes in every region around the globe. A broad range of indicators are telling us that we are witnessing rapid changes in many aspects of our global climate:

1. The Arctic Sea ice has seen a long-term decline since the beginning of the satellite era (1979-present). In September 2020, the Arctic sea-ice extent reached its second lowest minimum on record. The area and thickness of the Antarctic sea-ice extent also dropped rapidly in the three years following 2015. Greenland and Antarctic ice sheets are shrinking as the vast majority of glaciers worldwide
2. Global mean sea levels rose 20 cm from 1900 to 2018 at an accelerating rate of 3.7 mm/yr from 2006 to 2018. The global warming is causing thermal expansion of ocean water and melting land-based ice. The rate of ice sheet loss in Greenland and Antarctica increased by a factor of four between 1992-1999 and 2010-2019, thus constituting the dominant contribution to sea level rise
3. Hot extremes (including heatwaves) have become more frequent and more intense across most land regions since the 1950s, while cold extremes (including cold waves) have become less frequent and less severe. Marine heatwaves have approximately doubled in frequency since the 1980s
4. The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis. Agricultural and ecological droughts have increased due to the increased land evaporation
5. A global proportion of major (Category 3–5) tropical cyclone occurrence has increased over the last four decades, and the latitude where tropical cyclones in the western North Pacific reach their peak intensity has shifted northward
6. Human influence has likely increased the chance of compound extreme events since the 1950. This includes increases in the frequency of concurrent heatwaves and droughts on the global scale; fire weather in some regions of inhabited continents and compound flooding in some locations.

The near-term predictions for the coming five years (2021-2025) show a high probability for average global temperature to be between 0.9 and 1.8°C above pre-industrial conditions, with a 40% chance of at least one year exceeding 1.5°C above pre-industrial level. It is important to note, that this doesn’t mean that we fail the Paris Agreement 1.5°C goal - which refers to the climatological condition over a long-term average. Instead, this metric shows the increasing likelihood of a temporary exceedance of the 1.5°C temperature level as the climate warms, which is likely to occur as Earth’s climate draws closer to the temperature target set by the Paris Agreement.

Looking at climate futures the headline statements of the latest IPCC assessment report provide several reasons for concern:

• Global surface temperature will continue to increase until at least the mid-century under all emissions scenarios. Global warming of 1.5°C and 2°C will be exceeded during the twenty-first century unless deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades

• Many changes in the climate system become larger in direct relation to increasing global warming. They include increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic se ice, snow cover and permafrost

• Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation and the severity of wet and dry events. Under scenarios with increasing CO2 emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere.

• Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level

• From a physical science perspective, limiting human induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained reductions in methane emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.

Socio-economic impacts of climate change

All available evidence taken into consideration, including physical understanding, meteorological observations as well as different regional climate models, give high confidence that human-induced climate change has increased the likelihood and intensity of such an event to occur and these changes will continue in a rapidly warming climate. Under future climate change, such events will occur more frequently.

The recent WMO Atlas of Mortality and Economic Losses from Weather, Climate and Water Extremes [3] shows that of the 22 326 disasters recorded worldwide from 1970 to 2019 over 11 000 were attributed to weather, climate and water-related hazards. Those disasters resulted in 2.06 million deaths and US$ 3.64 trillion in losses. Some 44% of the disasters were associated with floods (riverine floods 24%, general floods 14%) and 17% with tropical cyclones. Tropical cyclones and droughts were the most prevalent hazards with respect to human losses, accounting for 38% and 34% of disaster related deaths respectively. In terms of economic losses, 38% were associated with tropical cyclones, while different types of floods account for 31%, riverine floods (20%), general floods (8%) and flash floods (3%) (WMO, 2021). Although the number of recorded disasters rose by a factor of five over the period the number of deaths decreased almost three-fold. This is due, in part, to better multi-hazard early warning systems, which are improving prevention, preparedness and response.

Of all of deaths, 91% occurred in developing economies according to the United Nations country classification. The proportion remains similar for the World Bank country classification, according to which 82% of deaths occurred in low and lower-middle income countries. The two different economic classification methodologies – the United Nations and the World Bank – both reveal that the majority of reported deaths from weather, climate and water extremes occurred in developing countries, while countries with developed economies incurred the majority of economic losses.

It is also important to understand the connections between climate change and the Sustainable Development Agenda for 2030 far beyond the Sustainable Development Goal (SDG) 13 for climate action. A recent study [4] on the implications of the latest data and scientific research on the state of the global climate for sustainable development highlighted how our changing climate is already affecting the achievement of the SDGs. Because CO2 concentration drives global climate change, it is indirectly responsible for risks related to nearly every single SDG.

Based on seven climate indicators (surface temperature, ocean heat content, atmospheric carbon dioxide concentrations, ocean acidification, sea level rise, glacial mass balance and sea ice extent), the study concluded with some degree of confidence that climate change poses dire risks to sustainable development, most notably for biodiversity, resulting in potential challenges for achieving food security (SDG 2), tackling poverty (SDG 1), and maintaining peace (SDG 16). The cascading effect of increasing CO2 concentration on the achievement of sustainable development goals is reinforcing existing inequalities and threaten basic needs such as food, water, health, shelter, economic security and collective peace worldwide.

Understanding the complexities of climate change and international development is an ongoing challenge. There is the need, however, for greater international collaboration, which is essential for achieving the SDGs, and for limiting global warming to less than 2°C or even 1.5°C by the end of this century. Undoubtedly, an acceleration in reducing GHGs emissions is one of the most effective and necessary climate-related actions for achieving the SDGs.

Urgent call for action

At the time of writing this article, the most recent analysis of the global carbon dioxide emissions provides some troubling evidence that comes as no surprise: global fossil CO2 emissions in 2021 are set to rebound close to their pre-COVID levels. Such rebound in 2021 reflects a return towards the pre-COVID fossil-based economy and put in evidence that investments in the green economy in the post-COVID recovery plans have been insufficient to set an emission reduction trend.

Despite that climate change poses a huge global challenge, it is also a huge opportunity. Already, the last decade has seen a big reduction in key renewable costs such as solar, wind and battery storage, with strong evidence of benefits reaching other parts of the economy.

Across all sectors of the global economy there are vast opportunities to support creative solutions. With great determination and courage, a clean and sustainable future could await, generated by technological innovation, support for nature and the services it provides, and by each and every societal player striving for an improved future.

Global leaders must take responsibility at COP26 to embrace the change and the opportunities that can emerge. We cannot miss the chance to secure a multigenerational legacy for future generations.

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