The connection between human emissions of CO2 and observed changes in the climate is clear, but the consequences and overall extent of these changes can be much harder to pin down. Projections from climate models currently put average warming between 2.2 °C and 3.5 °C by the end of this century (following current emission rates and existing policy from 2021). When every tenth of a degree matters, this wide range leaves a lot to the unknown. Why is it so challenging for scientists to predict the consequences of changes to the climate? One reason is climate feedbacks.
Climate feedbacks are the natural response to a change in the earth system. They can happen over hundreds, thousands, or millions of years following cycles in solar activity, volcanic eruptions, or from wildfires and floods over shorter timescales. As humans rapidly emit significant amounts of CO2 and other greenhouse gasses into the atmosphere, more heat is trapped. This causes average temperatures to rise, triggering a comparatively swift response from environmental systems. These feedbacks can, in turn, either stabilize or amplify the initial change.
Stabilizing feedback loops are regularly referred to as negative feedbacks, and amplifying feedbacks as positive. This terminology can be confusing, so we will keep with the more straightforward phrasing. An example of a stabilizing feedback is lapse-rate. The atmosphere gets colder with increasing altitude, so as the surface warms, excess heat gets bled off, cooling the surface.
Feedbacks are behind the increase in extreme weather events, droughts, wildfires, and so forth from climate change, which can trigger subsequent feedbacks. If enough amplifying changes build-up to the point of being irreversible, a tipping point is reached with large consequences to the climate system. Think of a glass of water; once you knock it over, you can fix the glass but can’t put the spilled water back. Scientists do not think we have reached any major tipping points yet, but many concerning amplifying feedbacks are being observed, with the full extent of their effects still unclear.
Some of the most-relevant climate feedbacks include:
Water vapor. Warmer temperatures increase the rate of evaporation from the surface and the atmosphere’s capacity to hold water. As a greenhouse gas, more water vapor will amplify warming.
Clouds. Clouds reflect a third of incoming solar radiation away from the planet, so even small changes in their size, type, and location affect the climate. More water vapor in the atmosphere can increase the number of clouds, but the specific type is still being studied and is highly regionally dependent. Varying cloud properties can change whether they have a cooling or warming effect over time. The difficulty of modeling individual clouds and representing all cloud processes makes them one of the largest uncertainties in climate projections. The current research suggests they have an overall amplifying effect.
Snow- and ice-albedo. The bright and reflective properties (called albedo) of snow and ice absorb less heat than dark surfaces like the ocean and land surface. As snow and ice melt away, more heat is absorbed, causing more melting. This amplifying feedback is one reason why warming in the Arctic is happening at a faster rate than in lower-latitude regions.
Permafrost. Around 25% of the northern hemisphere is made up of frozen soils called permafrost. Increasing average temperatures thaw these soils out, releasing CO2 and methane as the large quantities of previously frozen organic matter decompose. Methane is particularly concerning because it’s 120 times stronger at trapping heat than CO2. Over 100 years, a kilogram of methane can retain 36 times the amount of heat compared to a kilogram of CO2. Permafrost-thaw is the other main reason higher latitudes are warming faster than anywhere else. The ratio of CO2 to methane released by permafrost varies by local conditions and is still being studied.
Forests. Plant growth on land and in the oceans absorbs around half of all CO2 emissions from humans per year. Warmer temperatures and greater concentrations of atmospheric CO2 could lower this increased uptake over time, reducing these natural carbon sinks. Changing regional conditions, including the increased magnitude and frequency of droughts, pest outbreaks, and wildfires, reduces forest health, leading to less carbon being stored and more warming.
These are only a select few examples, as many more exist. Much uncertainty in climate models comes from the exceptional complexity of the numerous earth system feedbacks. Without feedbacks, our ability to predict changes in the climate is easy. We know that doubling atmospheric CO2 concentrations compared to 1750 would increase average temperatures by 1.2 °C by 2100. It gets complicated when you factor in all the subsequent changes triggered by this initial change. Scientists have made remarkable progress in narrowing the range of potential warming over the last decade, but a precise outcome cannot be guaranteed.
Tipping points are challenging to identify before they happen. Often, by the time they are recognized, it’s already too late. The observed rate of ice loss in the Arctic is much faster than climate models have previously predicted and are likely to reach a point of no summer sea ice in the next decade or two. The Amazon rainforest is one of the most biodiverse ecosystems on the planet. Still, drier temperatures, more wildfires, and deforestation by humans make it plausible for this massive reservoir of carbon to tip and disappear by 2100. Currently, the Amazon may already be emitting more CO2 than it stores. While recent reports are confident some of the more dire tipping points are less likely to happen—such as a collapse of the Atlantic Meridional Overturning Circulation Current—they can’t be fully taken off the table.
The most considerable feedback representing uncertainty in future climate is us. Human activity, from our emissions of greenhouse gasses to our land-use practices, triggers feedbacks across all elements of the earth system. While many concerning positive feedback loops exist, immediate action can still slow the cycle of amplification and alleviate some of the worst impacts.
Learn more about feedbacks and tipping points by exploring these resources:
- 15 Climate Feedback Loops and Examples (Earth How)
- Feedbacks and tipping points: Big uncertainties about future warming (E&E News)
- Guest Post: Understanding climate feedbacks (Carbon Brief)
- How feedback loops are making the climate crisis worse (Climate Reality Project)
- Problem solving activity: Climate change and feedback loops (NOAA)
- The Study of Earth as an Integrated System (NASA Global Climate Change)
- When nature harms itself: Five scary climate feedback loops (Deutsche Welle)
- Why Positive Climate Feedbacks Are So Bad (World Resources Institute)