May 13, 2021
A Guide to Climate Tipping Points: The Weather
Making sense of a world on edge
This article is part of a series:
- Introduction: What are tipping points?
- 1. The Arctic
- 2. The Antarctic
- 3. The Oceans
- 4. The Weather
- Reflection: Where to start?
4. The Weather
The oceans, land, and atmosphere interact in complex ways that drive changes in air circulation, temperature, and humidity over our continents. Climate change is expected to drive more extreme weather, but that will look different depending on where you are in the world. We have seen how the polar jet stream affects weather throughout North America. Other major drivers include the El Niño system over the Pacific Ocean, monsoon systems across land-ocean boundaries, and local hydrological cycles maintained by forests and mountains.
4.1 More Permanent El Niño State
How does the system normally function? Over the equatorial Pacific Ocean, trade winds move water from east to west, piling up hotter water in the western Pacific and drawing up cooler water in the eastern Pacific. In a cycle that lasts about four years, the winds speed up and slow down, shifting the location of the hot water. When the hot water moves east, El Niño conditions occur, driving extreme wet and dry weather in different parts of the world, partly through displacement of the polar jet stream (see #1.9). When the hot water moves west, La Niña conditions occur, with opposite weather effects. The coupled water-atmospheric system is known as the El Niño–Southern Oscillation (ENSO).
How is it affected by climate change? The interactions between ENSO and climate change are complex, but most scientists agree that shifting surface temperatures and atmospheric dynamics will have an effect. One study suggests that the ocean surface will warm particularly over the eastern Pacific, due in part to disruptions to ocean circulation patterns even in the Atlantic (see #1.3). This would increase the temperature gradient in El Niño events, making them more intense. Locations of rainfall are also likely to shift.
How does this affect the planet? About every 20 years, a particularly extreme instance of El Niño or La Niña will occur, but climate models suggest that, with rising temperatures, extreme events are likely to occur more frequently. The effects would be experienced around the globe, including dryer monsoon rainy seasons in India (see #4.4) and in the Sahel (see #4.3).
What are the tipping points? Current climate models do not demonstrate tipping point behavior in ENSO shifts, suggesting they could be gradual and reversible.
What is the timeline? Studies suggest El Niños are already becoming more intense and that the frequency of extreme events could increase to every 10 years by 2100, even under a 1.5 °C global temperature increase.
What can we do about it? Reducing atmospheric greenhouse gas concentrations can reduce the warming effects driving ENSO shifts. But in all parts of the world, communities should prepare for more extreme cases of the weather they have come to expect from El Niño and La Niña.
4.2 Dieback of the Amazon Rainforest
How does the system normally function? The Amazon Rainforest serves multiple benefits to the planetary system. It is one of the most biodiverse ecosystems in the world, as well as a major carbon sink, containing about 15% of total carbon stored in global vegetation and drawing down about 5% of annual carbon emissions. Through transpiration, it creates reflective clouds and draws air from the ocean, pouring rain over the South American continent.
How is it affected by climate change? The primary driver of Amazon dieback is land use practices encouraging the active destruction of rainforest habitat for agriculture and livestock. However, the ecosystem is also impacted by climate change dynamics under counterbalancing forces. On one hand, warming and drying diminish rainforest viability. However, increased atmospheric CO2 accelerates photosynthesis and makes it more water-efficient, albeit with less transpiration and, therefore, a slower water cycle. Precipitation is also influenced by shifting El Niño cycles (see #4.1), and nutrient cycles depend on the future of the Sahara (see #4.3). So far, about 11 percent of the rainforest is considered deforested and another 17 percent degraded. Findings also indicate that the rainforest is getting wetter, while deforested areas are getting drier.
How does this affect the planet? Reduced forest cover and viability restrict the system’s effectiveness as a carbon sink, both releasing stored carbon into the atmosphere and shrinking its drawdown capacity. Disturbance of the water cycle will lead to drought on the South American continent. Mass extinction would result in the biodiversity hotspot, reducing variation in the genetic pool and, therefore, overall resilience of the biosphere.
What are the tipping points? A tipping point has been predicted for deforestation at 20–25% of land cover, triggering a shift to a savanna ecosystem over large swaths of the region and converting from a carbon source to a carbon sink. Recent droughts and floods appear to indicate that the ecosystem is beginning to oscillate between two states. In the absence of deforestation, global warming could trigger a similar transition with a temperature increase of 4 °C.
What is the timeline? Deforestation was brought under control by Brazilian policy in 2011, but under the leadership of Jair Bolsonaro, it is again on the rise. If trends continue, some predict the tipping point could be reached in 10–15 years.
What can we do about it? The most urgent action is to protect and restore the Amazon rainforest ecosystem. Jair Bolsonaro has refuted international pressure as a threat to Brazilian sovereignty, but the US is in talks to make a deal.
4.3 Greening of the Sahara/Sahel
How does the system normally function? Between Africa’s arid Sahara Desert and Sudan-Guinea Savanna lies a thin transition region called the Sahel. The climate of these areas is governed by the West African monsoon, a seasonal cycle of alternating winds driven by differences in humidity between the Sahara and the Atlantic Ocean. Typically, the monsoon drives alternations between a dry season with lower vegetation and a wet season with higher vegetation.
How is it affected by climate change? Paleoclimate studies suggest that the behavior of the West African monsoon has been highly variable throughout history, including a period where the Sahara Desert was wet and vegetated from 12,000 to 5,000 years ago. More recently, in the 1970s, an increase in ocean surface temperature caused drought in the Sahel, offering a glimpse of changes to come. As land heats up faster than ocean from global warming, the Sahel and southern parts of the Sahara could see more rain, driving new, stable vegetation. However, there are counterbalancing forces. Atmospheric aerosols from human pollution could dampen the increase in land temperature, and slowing ocean currents could lead to faster increases in water temperature (see #1.3).
How does this affect the planet? Greening the Sahel and Sahara could have positive effects on the bioregion, including increased biodiversity and ecosystem services for the region, as well as serving as a new carbon sink. However, the change would come with ripple effects on other ecosystems. A major factor is dust. Huge volumes of nutrient-rich dust from the Sahara’s historical green periods move over the Atlantic Ocean each year, adding carbon to the ocean, decreasing tropical cyclone activity, and fertilizing the Amazon Rainforest. Increasing rainfall would mean less dust, disrupting the marine carbon pump (see #3.1), El Niño cycles (see #4.1), and the Amazon Rainforest (see #4.2).
What are the tipping points? Based on paleoclimate evidence, we know that tipping points can bring about rapid change, alternating the region between desert and savannah ecosystem.
What is the timeline? Given the competing forces, models do not predict a high confidence in reaching a threshold under a 2 °C global increase.
What can we do about it? Many see a greening Sahel as a net positive, and efforts to terraform the region are already underway. A major initiative to plant trees across the Sahel known as the Great Green Wall initially failed due to an inhospitable climate and insufficient management, though it has evolved into more localized and intensive reforestation efforts focused on restoring small water cycles. Models suggest that an engineered forest in the Sahara would only drive 15–20% of the rainfall it would need to survive.
4.4 Chaotic Indian Summer Monsoon
How does the system normally function? The Indian monsoon functions similarly to the West African monsoon, whereby seasonal differences in temperatures between the land and sea drive alternating wet and dry seasons. The monsoon delivers about 70 percent of India’s annual rainfall. Because the monsoon trough shifts north and south around the foothills of the Himalayas, rain in each location can vary dramatically throughout the summer wet season.
How is it affected by climate change? Similarly to the West African monsoon, competing factors are at play. Increasing land temperatures could increase rainfall, while aerosols from pollution could dampen the effect. This could possibly lead to increased variability and extremes. Increased humidity from rising air temperatures could make wet seasons longer and more intense, though this would decrease during more extreme El Niños (see #4.1). One study suggests that monsoon rainfall has already been increasing since 2002.
How does this affect the planet? Chaotic monsoon activity is likely to impact India’s population, including increased flooding and disease, as well as disrupted food production. For example, during the 2018 record wet season, thousands of people died, and 2.5 million people were injured or displaced. An extreme heat wave in 2019 was exacerbated by a delayed wet season, causing hundreds of deaths.
What are the tipping points? Scientists debate whether the system has clear tipping points. Some propose that monsoons have stable active and dry states, while others claim that the spectrum of activity is linear and reversible.
What is the timeline? Predictions are mixed, with some reports predicting no change under a 2 °C temperature increase and others predicting increased precipitation and variability over the course of the century.
What can we do about it? Reversing the trends of rising temperatures and air pollution could help stabilize the Indian monsoon. However, it is likely that India will need to prepare to mitigate the effects of flooding. This could include creating embankments and dams to control floods, planning cities with ponds and permeable surfaces, restoring wetlands and forests, and regulating development in floodplains and hills.
4.5 Melting of the Himalayan Glaciers
How does the system normally function? The Hindu Kush-Himalaya mountain system separates the plains of the Indian subcontinent from the Tibetan plateau, bordering 9 countries. Meltwater from snow and glaciers feeds several major river systems, which provide drinking water and ecosystem services to nearly 2 billion people. The meltwater is replenished by snow, driven by seasonal activity in the Indian and East Asian monsoon systems (see #4.4) and varying greatly across its complex topography.
How is it affected by climate change? The processes that drive Arctic amplification similarly drive a phenomenon known as elevation-dependent warming (see #1.1). The highest elevations of the Himalayas are seeing accelerated warming due to a feedback loop: elevated temperatures melt snow and ice, reflectivity decreases, and atmospheric water vapor increases, increasing atmospheric heating further. The effects of moistening are pronounced at higher elevations, which are currently driest. Aerosols, particularly black carbon and dust, encourage more rapid glacial melt, similar to the Greenland ice sheet (see #1.2). Recent reports indicate the rate of glacial melt has doubled since 1975, totaling a 15% loss of volume.
How does this affect the planet? A melting Himalaya could disrupt the lives of billions of people in terms of water, food security, natural disaster, and forced migration. The bioregion is also home to multiple biodiversity hotspots, such as mountain forests, alpine meadows, and wetlands, that could migrate, shrink, or collapse. Globally, the melt will contribute to sea level rise.
What are the tipping points? Parts of the Hindu Kush Himalaya range have already crossed a threshold into net mass loss. Multiple current and potential dynamics appear difficult to reverse, including upward migration of boundaries with permanent snow cover and transitions in ecosystem states.
What is the timeline? A recent study estimated glacier volume loss by 2100 at one-third at low emissions (1.5 °C increase), one-half at moderate emissions (2 °C increase), and two-thirds at high emissions (4–5 °C).
What can we do about it? Reducing atmospheric greenhouse gases can slow down melting. Preparing for inevitable changes will require transnational collaboration in a region of political tension. Proposals include improving mountain ecosystem resilience and disaster monitoring and response infrastructure.
4.6 Aridification of Southwest North America
How does the system normally function? The climate of the North American Southwest is typically characterized by warm and dry conditions. This is largely a result of its position between the polar jet stream and subtropical jet stream, where the northern atmospheric Hadley cell tends to deposit dry air. Historical records show variation in aridity as a result of natural oscillations in Pacific and Atlantic ocean temperature, including frequent decades-long “megadroughts” between the years 900 and 1600.
How is it affected by climate change? Climate models forecast an expansion of the Hadley cell, which would intensify heat and aridity in the region. This increase is reinforced by positive feedback: rising temperatures mean the air will tend to suck up more moisture, drying out water bodies and soils. Snowfall will also give way to rains, decreasing surface reflectivity and further increasing temperature. Drought could also be intensified by more frequent extreme La Niña states (see #4.1).
How does this affect the planet? Local impacts have already been observed, including heat waves and wildfires in California, diminishing water reserves in Arizona, lack of mountain snow in Washington, and drought in Utah. Climate models predict worsening and lengthening drought conditions, impacting water supply, food production, and the stability and carbon cycle of ecosystems.
What are the tipping points? Some level of aridification is thought to be irreversible on human timescales. Local tipping points in the ability of soil to retain moisture, the ability of mountains to replenish ice, or the capacity of ecosystems to restore to a pre-drought state would be difficult to reverse.
What is the timeline? A recent study estimated the likelihood of multidecade droughts in the southwest by the year 2100 at 75% under moderate emissions (2 °C increase) and over 80% at high emissions (4 °C increase).
What can we do about it? Diminishing atmospheric greenhouse gas concentrations is, once again, a critical factor. Communities in the North American Southwest can also prepare for increased drought and fire by managing water sources and use, recycling water through “sponge cities,” cultivating small water cycles, adopting drought-resistant agricultural approaches, improving management of and response to fires, and cultivating ecosystems to be more drought-resistant.
This article is part of a series:
