Scientists simulate climate’s worst case scenario

What happens if the human population continues to grow and nothing much changes in the way we curb fossil fuel use?

Climate models these days have largely focused on scenarios that assume some level of restraint on greenhouse emissions, with particular emphasis on the political goal of keeping global temperatures no higher than 2 degrees above pre-industrial levels.  But scientists at the National Center for Atmospheric Research in Boulder and the Institute for Atmospheric and Climate Science in Zurich have simulated scenarios that show the upper boundaries of future greenhouse gas emissions.

In a paper published recently in the journal Environmental Research Letters, the scientists write that understanding these upper range scenarios is crucial for good decision-making about climate change.

In one climate scenario, the human population grows from 6 to 11 billion by the end of the century, while energy supplies gradually shift from today’s heavy reliance on fossil fuels to 30 percent from carbon neutral sources (compared to 14 percent in 2000). The higher demand for energy results in 55 Gigatons of carbon per year by 2100, which is similar to the highest nonintervention emissions scenarios in existing published literature. In other words, more and more people on Earth will more than erase the gains made in shifting to cleaner energy streams.

The second scenario is meant to represent a worst case situation. The population reaches 15 billion by 2100 and all the extra energy needed comes from coal, the fossil fuel with the highest amount of carbon per unit of energy. The result is emissions reaching 105 Gigatons a year.

In running the simulations, the main differences in global mean temperatures between the two scenarios doesn’t become apparent until the second half of this century. But the impacts can be seen quite starkly. Under an All-Coal scenario, 90 percent of Arctic sea ice is gone from August through October by 2060, while it takes until 2075 for sea ice to disappear in the first scenario.

From there things get steadily worse in the All-Coal scenario. Sea level increases by 8 feet over 1990 levels by 2500. Oddly enough, until the year 2100 sea levels remain nearly constant, even under no change in greenhouse gases because of the thermal inertia of the oceans.

Global temperatures shoot up to 14 degrees Fahrenheit above 1990 levels. Some parts of the planet — like the Southern Europe, Central America, and ocean subtropical regions – experience as much as an 80 percent drop in precipitation above late 20th century levels. Other regions – notably the Arctic and Antarctic – get inundated with as much as a 200 percent increase in rainfall. The Amazon basin gets hammered with a 20 percent increase in rainfall under all future scenarios.

Granted, All-Coal is the worst that can happen, and presumably (or not?) humans take action, or use up all coal reserves, before the devastation hits. But it’s a scenario worth pondering. After all, there is an influential mindset that supports using up all known coal reserves and favors unchecked population growth. This is the world they would create.