In order to see how climate models react over a wide range of greenhouse gas concentrations, researchers in the US have modelled emissions scenarios that are significantly higher than the IPCC’s “worst case” scenarios. They found – perhaps unsurprisingly – that the extent of climate change will be significantly worse than for the IPCC’s A1FI scenario.

“Relative to the A1FI scenario, our highest scenario results in an additional 2°C (3.6F) of global mean warming above A1FI levels by 2100, a complete loss of Arctic summer sea ice by 2070 and an additional 43% sea level rise due to thermal expansion above A1FI levels by 2100,” said Ben Sanderson from the National Center for Atmospheric Research in the US.

He told environmentalresearchweb: “Our aim was not to be fatalistic, but to use these extreme scenarios in order to better understand the global climate model and the climate system. We felt that, though the scenarios we used may not entirely be plausible, it would be prudent to consider the upper bounds of the range of future emissions scenarios and their potential consequences, as well as the lower bounds which have been already studied in detail.”

The first scenario (CurrentMix) adopts the global per capita primary energy outcome of the IPCC’s A1FI scenario and combines it with the UN medium population projection assumed in the IPCC’s B2 scenario, which foresees an increase from 6 billion people in 2000 to 11 billion by the end of the century. The team also assumed that the shares of primary energy derived from different fuel sources remain fixed over time at 2000 levels; that is, the carbon intensity of energy supply is assumed to remain constant.

In the second scenario (AllCoal), the researchers make more extreme assumptions. They maintain the A1FI per capita energy projection, but assume population follows the UN high scenario as implemented in the IPCC A2 scenario, reaching 15 billion by 2100. They also make the bounding assumption that all new demand for primary energy is satisfied by coal.

“This assumption is not intended to represent a plausible future, but a useful thought experiment that could help inform the exploration of upper bounds on emissions,” said Sanderson. “It is astounding, for example, that this combination of assumptions leads to emissions in 2100 that are about four times those in the A1FI scenario, or about 105 gigatonnes of carbon per year.”

The conclusion of the modelling work was that although the simulations reveal no significant nonlinearities in global climate feedbacks, significant additional climate changes would occur in a hypothetical future without any form of climate mitigation or restriction on fossil fuel availability. Sanderson also points out that the results are dependent on a single model that is not capable of simulating feedbacks involving the Earth’s carbon cycle (one candidate for large nonlinearities in response to increasing emissions). He therefore believes that the next logical step is to repeat experiments of this type using a range of next-generation models with a fully interactive carbon cycle.

The scientists published their work in Environmental Research Letters (ERL).