For anyone who still thinks that climate change is a hoax, this is quite the wake up call. A new study evaluating models of future climate scenarios has led to the creation of two new risk categories: “catastrophic” and “unknown”. Let’s pause for a minute – two new categories had to be created to emphasise the trouble we are in. The researchers are simply saying that unknown risks imply existential threats to the very survival of humanity.

These categories describe two low-probability but significant scenarios that could play out by this century’s end. The risk assessment is based on the objective of the 2015 Paris Agreement – to keep average global temperatures “well below” a 2°C increase from what they were before the Industrial Revolution.

This study has been conducted by Veerabhadran Ramanathan, a distinguished professor of climate and atmospheric sciences at Scripps Institution of Oceanography at the University of California San Diego, and his former Scripps graduate student Yangyang Xu.


The impact of climate change on wildlife is well documented. Photo: Lwp Kommunikáció via Flickr

A rise of 1.5°C

The study says that even if that objective is met, a global temperature increase of 1.5°C is still categorized as “dangerous.” That means it could create substantial damage to human and natural systems.

A rise greater than 3°C

A temperature increase greater than 3°C could lead to what the researchers term “catastrophic” effects, and an increase greater than 5°C could lead to “unknown” consequences which they describe as beyond catastrophic, including potentially existential threats.

This reflects grave risks to human health and species extinction from warming beyond 5°C, which has not been experienced for at least the past 20 million years.

‘We’d never get on a plane with a one-in-20 chance of it coming down’

The scientists term warming probability of five percent or less as a “low-probability high-impact” scenario. Ramanathan and Xu also describe three strategies for preventing the gravest threats from taking place.

“The probability is equivalent to a one-in-20 chance the plane you are about to board will crash,” said Ramanathan. “We would never get on that plane with a one-in-20 chance of it coming down but we are willing to send our children and grandchildren on that plane.”

So what are the consequences of the deadly heat?

  • “Dangerous” global warming includes consequences such as increased risk of extreme weather. And climate events ranging from more intense heat waves, hurricanes, and floods, to prolonged droughts.
  • Planetary warming between 3°C and 5°C could trigger what scientists term “tipping points” such as the collapse of the West Antarctic Ice Sheet and subsequent global sea-level rise, and the dieback of the Amazon rainforest.
  • In human systems, catastrophic climate change is marked by deadly heat waves becoming commonplace, exposing over 7 billion people to heat related mortalities and famine becoming widespread. Furthermore, the changes will be too rapid for most to adapt to, particularly the less affluent, said Ramanathan.

The researchers defined the risk categories based on guidelines established by the Intergovernmental Panel on Climate Change (IPCC) and previous independent studies. Risk assessments of global temperature rise greater than 5°C have not been undertaken by the IPCC.  Ramanathan and Xu named this category “unknown??” with the question marks acknowledging the “subjective nature of our deduction.”

The existential threats could include species extinctions and major threats to human water and food supplies in addition to the health risks posed by exposing over 7 billion people worldwide to deadly heat.

sign climate change

Photo: Garry Knight via Flickr

What is the prevention strategy?

With these scenarios in mind, the researchers identified what measures can be taken to slow the rate of global warming to avoid the worst consequences, particularly the low-probability high-impact events.

Aggressive measures to curtail the use of fossil fuels and emissions of so-called short-lived climate pollutants such as soot, methane and HFCs would need to be accompanied by active efforts to extract CO2 from the air and sequester it before it can be emitted.  It would take all three efforts to meet the Paris Agreement goal to which countries agreed at a landmark United Nations climate conference in Nov 2015.

Is this attainable?

Xu and Ramanathan point out that the goal is attainable. Global CO2 emissions had grown at a rate of 2.9 percent per year between 2000 and 2011, but had slowed to a near-zero growth rate by 2015.

They credited drops in CO2 emissions from the United States and China as the primary drivers of the trend. Increases in production of renewable energy, especially wind and solar power, have also bent the curve of emissions trends downward. Other studies have estimated that there was by 2015 enough renewable energy capacity to meet nearly 24 percent of global electricity demand.


Photo: Jurgen via Wikimedia Commons

Short-lived climate pollutants are so called because even though they warm the planet more efficiently than carbon dioxide, they only remain in the atmosphere for a period of weeks to roughly a decade whereas carbon dioxide molecules remain in the atmosphere for a century or more. The authors also note that most of the technologies needed to drastically curb emissions of short-lived climate pollutants already exist and are in use in much of the developed world.  They range from cleaner diesel engines to methane-capture infrastructure.

“While these are encouraging signs, aggressive policies will still be required to achieve carbon neutrality and climate stability,” the authors wrote.

Source: Scripps Institution of Oceanography

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