[Everything in this post is true to the best of my knowledge, but I am not a climatologist and my last science class was in high school. It is very likely I have misunderstood something. I welcome corrections. Dedicated to Annie.]
I often see people say that climate change is going to kill us all, or that human extinction is inevitable due to climate change, or that if effective altruists were really concerned about existential risk we would put lots of effort into fighting climate change because all scientists agree that is the biggest existential risk.
As far as I can tell, absolutely none of this is true.
Climate Feedback, a nonpartisan website educating people about the scientific consensus on climate change, has written about human extinction from climate change. Christopher Colose, a postdoctoral research fellow at NASA GISS, puts it clearly:
Actual numbers are important here. The global temperature increase could indeed reach 4-5 degrees by 2100, if humans don’t do anything to our emissions, and beyond this patches of uninhabitable areas (for humans) could start to open up in the tropics, due to heat stress limits imposed by the evaporative limits of our body. Indeed, a world 5+ degrees warmer is a big cause for alarm, even if the world takes a linear path to that mark. The world also does not end in 2100, and while it is tempting to think of later dates as “very far off,” it is worth reminding ourselves that we would live on a different planet had people of the Viking era industrialized and emitted carbon uncontrollably.
Nonetheless, the near future climatic fate of New York probably looks more like the climate of South Carolina or Georgia than something from a Mad Max movie. This is still an important basis for concern given that the socio-political infrastructure that exists around the world is biased toward the modern climate.
Many of the nightmare scenarios in this article, such as no more food, unbreathable air, poisoned oceans, perpetual warfare, etc. are simply ridiculous, although food security is indeed an issue at stake (see David Battisti’s comments). A “business-as-usual” climate in 1-2 centuries still looks markedly different than the current one, but there’s no reason yet to think much of the world will become uninhabitable or look like a science fiction novel.
The IPCC, the world’s leading experts on climate change, publish regular reports about climate change which explain what the expert consensus about climate change is. The most recent report about the impacts of climate change was published in 2014. (The fifth assessment report is in drafts but has not been officially published.) It lists five reasons for concern about climate change:
Unique and threatened systems— Climate change will destroy certain vulnerable cultures and ecosystems.
Extreme weather events— Climate change increases the risk of some natural disasters, such as heat waves, coastal flooding, and extreme amounts of rain.
Distribution of impacts— Climate change will disproportionately harm the global poor.
Global aggregate impacts— Due to climate change, we’ll have a harder time getting the things we usually get from nature– pollination, flood control, bushmeat, and so on– which will harm the economy.
Large-scale singular events— Some physical systems and ecosystems are at risk of abrupt and irreversible changes, such as the loss of the Greenland ice sheet.
Key risks are the potentially severe adverse consequences for humans resulting from climate change. The list of key risks is as follows:
i) Risk of death, injury, ill-health, or disrupted livelihoods in low-lying coastal zones and small island developing states and other small islands, due to storm surges, coastal flooding, and sea level rise. [RFC 1-5]
ii) Risk of severe ill-health and disrupted livelihoods for large urban populations due to inland flooding in some regions. [RFC 2 and 3]
iii) Systemic risks due to extreme weather events leading to breakdown of infrastructure networks and critical services such as electricity, water supply, and health and emergency services. [RFC 2-4]
iv) Risk of mortality and morbidity during periods of extreme heat, particularly for vulnerable urban populations and those working outdoors in urban or rural areas. [RFC 2 and 3]
v) Risk of food insecurity and the breakdown of food systems linked to warming, drought, flooding, and precipitation variability and extremes, particularly for poorer populations in urban and rural settings. [RFC 2-4]
vi) Risk of loss of rural livelihoods and income due to insufficient access to drinking and irrigation water and reduced agricultural productivity, particularly for farmers and pastoralists with minimal capital in semi-arid regions. [RFC 2 and 3]
vii) Risk of loss of marine and coastal ecosystems, biodiversity, and the ecosystem goods, functions, and services they provide for coastal livelihoods, especially for fishing communities in the tropics and the Arctic. [RFC 1, 2, and 4]
viii) Risk of loss of terrestrial and inland water ecosystems, biodiversity, and the ecosystem goods, functions, and services they provide for livelihoods. [RFC 1, 3, and 4]
To be clear, all of this is really bad. People are going to die. “Economic damage” sounds dry and abstract, but it means people going hungry and children dying of preventable diseases. The effects will fall substantially on the global poor, who have limited resources to help them cope. We can expect a climate refugee crisis.
It is also, notably, not human extinction.
Human extinction, if it were a likely outcome of climate change, would probably fall under reasons for concern #4 or #5 (although it’s still weird it didn’t get its own headlining concern). For this reason I will discuss #4 and #5 in more depth.
There is a low level of consensus about concern #4. If global warming is kept below 3 degrees Celsius, 20 to 30% of species are at risk of extinction– mostly specialist species who would have a far more difficult time adapting to climate change than humans would. Below 2.5 degrees Celsius, climate change will only cause a small reduction in gross world product; there is no consensus about the effects about 2.5 degrees Celsius, but they are expected to accelerate with increasing temperature. Human extinction is nowhere listed as a potential consequence of extreme levels of warming.
Concern #5 includes the following large-scale singular events:
Over centuries or millennia, the Greenland ice sheet and possibly the West Antartic ice sheet may deglaciate, leading to a five to ten meter rise in sea level.
The disappearance of the Arctic summer sea ice, which is likely reversible, although loss of biodiversity is not.
Irreversible changes in coral reef, Arctic, and Amazon ecosystems.
Two events described as “unlikely” or “very unlikely,” which include:
Accelerated carbon emissions from wetlands, permafrost, or ocean hydrates, which could cause a higher-than-predicted rate of global warming. I have not been able to find an estimate of how high the warming would be or what the effects would be.
Shutdown of the Atlantic meridional overturning circulation (AMOC), which would cause cooling in North America and Europe, increased extreme weather events, and (no matter what the Day After Tomorrow tells you) not human extinction
One concern I’ve often seen is an accelerated greenhouse effect turning us into Venus. However, Climate Feedback quotes Ken Caldeira, senior scientist at the Carnegie Institution for Science:
The Earth is not at risk of becoming like Venus. We have done climate model simulations in which all available fossil fuels were burned and the resulting CO2 released into the atmosphere. The planet warmed up about 10 °C in these simulations. This was enough to melt all of the ice sheets and produce 60 meters of sea-level rise, but in no such simulation does the Earth become anything like Venus.
Most of our climate models only extend to the relatively near future, such as 2100. There is a high degree of uncertainty about what will happen in the more distant future, with some exceptions, such as the melting of the Greenland ice sheet. It’s difficult to model things that are that far away. No one really knows what the world would look like in 2500 with three degrees of warming. But there’s also a lot of time in the next five hundred years to deploy other mitigation and adaptation solutions.
In conclusion, climate change will be very very bad. Lots of people will die. Many people– disproportionately the global poor– will go hungry, get sick or injured, not have access to clean water, or suffer from treatable or preventable illnesses. There will be many natural disasters. There will be global geopolitical instability, perhaps including wars and refugee crises. We will damage or lose many ecosystems that people value, such as coral reefs. The Amazon may become a grassland. But scientific consensus is that it will not result in human extinction or Earth becoming Venus.