The Basics of Climate Change

 

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 Are disaster scenarios about tipping points like “turning off the Gulf Stream” and release of methane from the Arctic a cause for concern?

Results from the best available climate models do not predict an abrupt change in (or collapse of) the Atlantic Meridional Overturning Circulation, which includes the Gulf Stream, in the near future. However, this and other potential high-risk abrupt changes, like the release of methane and carbon dioxide from thawing permafrost, remain active areas of scientific research. Some abrupt changes are already underway, such as the decrease in Arctic sea ice extent, and as warming increases, the possibility of other major abrupt changes cannot be ruled out.

The composition of the atmosphere is changing towards conditions that have not been experienced for millions of years, so we are headed for unknown territory, and uncertainty is large. The climate system involves many competing processes that could switch the climate into a different state once a threshold has been exceeded.

A well-known example is the south-north ocean overturning circulation, which is maintained by cold salty water sinking in the North Atlantic and involves the transport of extra heat to the North Atlantic via the Gulf Stream. During the last ice age, pulses of freshwater from the melting ice sheet over North America led to slowing down of this overturning circulation. This in turn caused widespread changes in climate around the Northern Hemisphere. Freshening of the North Atlantic from the melting of the Greenland ice sheet is gradual, however, and hence is not expected to cause abrupt changes.

Another concern relates to the Arctic, where substantial warming could destabilise methane (a greenhouse gas) trapped in ocean sediments and permafrost, potentially leading to a rapid release of a large amount of methane. If such a rapid release occurred, then major, fast climate changes would ensue. Such high-risk changes are considered unlikely in this century, but are by definition hard to predict. Scientists are therefore continuing to study the possibility of exceeding such tipping points, beyond which we risk large and abrupt changes.

In addition to abrupt changes in the climate system itself, steady climate change can cross thresholds that trigger abrupt changes in other systems. In human systems, for example, infrastructure has typically been built to accommodate the climate variability at the time of construction. Gradual climate changes can cause abrupt changes in the utility of the infrastructure—such as when rising sea levels suddenly surpass sea walls, or when thawing permafrost causes the sudden collapse of pipelines, buildings, or roads. In natural systems, as air and water temperatures rise, some species—such as the mountain pika and many ocean corals—will no longer be able to survive in their current habitats and will be forced to relocate (if possible) or rapidly adapt. Other species may fare better in the new conditions, causing abrupt shifts in the balance of ecosystems; for example, warmer temperatures have allowed more bark beetles to survive over winter in some regions, where beetle outbreaks have destroyed forests.

 

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If emissions of greenhouse gases were stopped, would the climate return to the conditions of 200 years ago?

No. Even if emissions of greenhouse gases were to suddenly stop, Earth’s surface temperature would require thousands of years to cool and return to the level in the pre-industrial era.

If emissions of CO2 stopped altogether, it would take many thousands of years for atmospheric CO2 to return to “pre-industrial” levels due to its very slow transfer to the deep ocean and ultimate burial in ocean sediments. Surface temperatures would stay elevated for at least a thousand years, implying a long-term commitment to a warmer planet due to past and current emissions. Sea level would likely continue to rise for many centuries even after temperature stopped increasing [Figure 9]. Significant cooling would be required to reverse melting of glaciers and the Greenland ice sheet, which formed during past cold climates. The current CO2-induced warming of Earth is therefore essentially irreversible on human timescales. The amount and rate of further warming will depend almost entirely on how much more CO2 humankind emits.

Scenarios of future climate change increasingly assume the use of technologies that can remove greenhouse gases from the atmosphere. In such “negative emissions” scenarios, it assumed that at some point in the future, widespread effort will be undertaken that utilises such technologies to remove CO2 from the atmosphere and lower its atmospheric concentration, thereby starting to reverse CO2-driven warming on longer timescales. Deployment of such technologies at scale would require large decreases in their costs. Even if such technological fixes were practical, substantial reductions in CO2 emissions would still be essential.