The views expressed in this report are those of the authors. Their statements are not necessarily endorsed by the affiliated organisations or the Global Challenges Foundation.

What is at stake?

Discussions of climate change typically focus on low- to mid-range scenarios, with temperature increase of 1°C to 3°C. These would have severe consequences, with potentially devastating effects on the environment and human societies. However, there is also a non-negligible and less often considered ‘tail-end’ risk that temperatures might rise even further, causing unprecedented loss of landmass and ecosystems. Global climate models indicate that even in a <2°C scenario, the most intense tropical cyclones become more frequent and more intense. In mid-range scenarios, entire ecosystems would collapse, much agricultural land would be lost, as would most reliable freshwater sources, leading to large-scale suffering and instability. Major coastal cities – New York, Shanghai, Mumbai – would find themselves largely under water, and the populations of low-lying coastal regions – currently more than a billion people – may need to be relocated. In high-end scenarios, the scale of destruction is beyond our capacity to model, with a high likelihood of human civilization coming to an end. 

How much do we know?

The Earth’s climate is impacted by the concentration of certain gases in the atmosphere, known as greenhouse gases, the most important being carbon dioxide and methane. As a result of human activity since the Industrial Revolution, the atmospheric concentrations of greenhouse gases – generally expressed as the number of greenhouse gas molecules per million or PPM – have risen consistently, reaching 400 ppm in 2015 and 403.3 ppm in 2016 from 280ppm at the dawn of the Industrial Revolution. When similar levels were last observed 3-5 million years ago, temperatures were 2-3°C warmer and sea levels 10-20 meters higher. Scientists had demonstrated an approximately linear relationship between the total amount of greenhouse gases emitted and the resulting temperature increase. There is now also a scientific consensus that climate change is a non-linear phenomenon where tipping points play a determining role. When warming rises above a certain level, self-reinforcing feedback loops set in, and the concentration of greenhouse gases increases rapidly. 

Although precise thresholds and exact scenarios remain uncertain, we know that the level of risk increases with the rise in temperature. The emissions pledge pathway negotiated at the Paris conference has a probability of over 90% to exceed 2°C, and only a ‘likely’ (>66%) chance of remaining below 3°C this century. In other words, even if current commitments were kept, there would remain a one-third probability of climate change in excess of 3°C – and we are presently not on track to meet the pledges.

At 3°C

If climate change was to reach 3°C, most of Bangladesh and Florida would drown, while major coastal cities – Shanghai, Lagos, Mumbai – would be swamped, likely creating large flows of climate refugees. Most regions in the world would see a significant drop in food production and increasing numbers of extreme weather events, whether heat waves, floods or storms. This likely scenario for a 3°C rise does not take into account the considerable risk that self-reinforcing feedback loops set in when a certain threshold is reached, leading to an ever-increasing rise in temperature. Potential thresholds include the melting of the arctic permafrost releasing methane into the atmosphere, forest dieback releasing the carbon currently stored in the Amazon and boreal forests, or the melting of polar ice caps that would no longer reflect away light and heat from the sun.

What are key factors affecting risk levels?

Climate change is a complex phenomenon affected by many factors. We may classify them into four categories to better discern the various areas where action is possible:

  • The risk is directly related to the release of greenhouse gases in the atmosphere through human activity. Carbon dioxide mainly results from the burning of fossil fuels for energy and transport. In turn, this is a factor of population growth and unsustainable production and consumption models. As to methane emissions, they largely relate to large-scale animal farming, driven by demand for meat, wool and dairy.
  • Some ecosystems store large amounts of carbon, particularly forests and coastal marine ecosystems, and their destruction could result in the large-scale release of greenhouse gases in the atmosphere.
  • The third factor is our capacity for global coordination to reduce emissions. This may be positively impacted by a better understanding of tail-end climate risk and climate tipping points, increasing the sense of urgency and prompting faster action.

  • Finally, the risk of catastrophic climate change is increased by insufficient knowledge and understanding of impacts and vulnerability, in turn affecting our ability to build resilience. The complex and interrelated nature of global catastrophic risk suggests an integrated research agenda to address related challenges and dilemmas – such as the use of solar geoengineering to reduce the risk of catastrophic climate change, which might harm in other ways – and keep human development safe.

After years of effort and considerable resources devoted to airplane safety, we have reached a point where 27 planes crash on average every year. If dying in a flight accident was as likely as a 3°C global temperature increase, then the number of people dying in airplanes every year would be 15,000,000.

Cities facing the highest risk from coastal flooding

Coastal cities are at particular risk from climate change, in developed and developing countries alike. This is of particular relevance as 1 billion people are currently estimated to live in coastal areas, lower than 20m above sea level, many of them in Asia. 

According to one study, taking the absolute estimated value of potential losses as a basis, the following cities face the highest risk from coastal flooding by 2050:  

  • Guangzhou, China 
  • Mumbai, India
  • Kolkata, India
  • Guayaquil, Ecuador
  • Shenzhen, China
  • Miami, USA
  • Tianjin, China
  • New York, USA
  • Ho Chi Minh City, Vietnam
  • New Orleans, USA

The risk of climate change for coastal cities can be measured in multiple ways. If we were to consider the increase in the level of risk, which may catch a city unprepared and cause sudden catastrophe, then, according to the same study, Alexandria, Barranquilla, Naples, Sapporo, and Santo Domingo face the greatest danger.


Why ice matters?

The Arctic region, mostly consisting of oceans, is covered with an ice sheet spanning about 14.4 million km² , or approximately half the size of Africa. Ice is reflective, and therefore absorbs less of the sun’s heat and energy. When it melts under the effect of climate change, to be replaced with open ocean, the amount of solar radiation reflected back to space is reduced, and the result is further warming of the planet.

Large quantities of water are also currently stored in frozen form on land – most of it over Greenland, Antarctica, and in mountain ranges as glaciers. It is predicted that approximately 1 meter of sea level rise from the melting of land ice is currently unavoidable, but things could get worse. If the entire Greenland ice sheet was to melt, it could potentially raise the world’s oceans by more than 6 meters. If all the ice currently standing on land and at the poles melted, at current estimates, sea levels would rise by more than 65 meters, flooding much of the planet’s inhabited land on all continents.

Displacement due to climate change

An important effect of climate change is an increase in the frequency and magnitude of extreme weather events - floods and storms principally - that affect the built environment, access to drinking water and other resources to support daily life, as well as social structures, and often result in the displacement of populations. Although precise attributions of causality can be complex, there is significant quantitative and qualitative data on past displacement associated with natural hazards and disasters. According to the Internal Displacement Monitoring Centre’s 2015 Global Estimates report, since 2008, an average of 26.4 million people per year have been displaced from their homes by disasters brought on by natural hazards, 85% of those weather-related. This is equivalent to one person displaced every second.

Civilizations lost to climate change

History records at least three instances of past civilizations collapsing under the local effects of climate change.

  1. Norse Viking settlers arrived and thrived in Greenland during the medieval warm period (800-1200 AD). When a period of cooling known as the Little Ice Age began in the early 14th century, it became increasingly difficult to farm. By the middle of the 16th century, the changing climate had contributed to the Vikings deserting their settlements and moving on to warmer lands.
  2. The Khmer Empire flourished from 802 to 1431. Its capital of Angkor Wat was one of the most ancient hydraulic cities, with a sophisticated system for irrigation to ensure optimal water reserves for the population’s growing needs. In the 14th and 15th centuries, decades of severe drought struck, interspersed with violent monsoon floods, bringing about political and social unrest which eventually led to the empire’s collapse. 
  3. From 3300 to 1700 BC, the Indus Valley Civilization developed sophisticated infrastructure and urban planning, and the population is estimated to have reached over 5 million. A 200-year drought that began around 2000 BC made agriculture unsustainable, and cities were gradually abandoned.

In all three instances, climate change was local, its cause was independent from human action, and the civilizations affected could not anticipate the change in their natural environment. The global nature of the climate change risk we face today bodes ill for humanity. If our civilization collapses on this planet, there is currently no alternative location where humanity may thrive. However, scientific and technological developments have made us more aware both of the risk we face, and of our influence on it. As a result, for the first time in history, we are in a position to reduce and possibly avoid the risk of civilization collapse due to climate change.

Leena Srivastava

Vice Chancellor, TERI University, New Delhi