The Most Likely Worst-case Scenarios For Climate Change
The most likely worst-case scenarios for climate change—often referred to as high-end warming pathways—are those where global temperatures rise significantly due to a combination of continued greenhouse gas emissions, slow policy responses, and feedback loops in the Earth system. These are not “guaranteed” outcomes, but they represent plausible futures if the world fails to curb emissions and adapt in time.
Here’s a breakdown of the top worst-case scenarios, globally and interconnected:
- 1. 4°C+ of Global Warming by 2100
- If emissions continue on a high-growth, fossil-fuel-heavy trajectory, we could see global average temperatures rise by 4°C or more by 2100.
- Consequences:
- Unlivable heat in many regions, especially parts of the Middle East, South Asia, and sub-Saharan Africa.
- Deadly heatwaves exceeding the threshold for human survival even in the shade, with wet-bulb temperatures >35°C.
- Major agricultural regions may become nonviable.
- Global GDP could shrink by up to 30% or more.
- Climate tipping points (see below) become more likely.
- 2. Triggering Irreversible Climate Tipping Points
- Climate systems have thresholds that, once crossed, can create runaway feedback loops. A worst-case scenario involves multiple tipping points being triggered:
| Tipping Element | What Could Happen |
|---|---|
| Greenland ice sheet | Irreversible melting → 7m sea level rise (over centuries) |
| West Antarctic ice sheet | Potential for rapid collapse → 3m sea level rise |
| Amazon rainforest dieback | Carbon sink turns into a carbon source |
| Permafrost thaw | Releases gigatons of methane—a super-potent greenhouse gas |
| Atlantic Meridional Overturning Circulation (AMOC) slowdown or collapse | Disrupts weather, monsoons, and food systems in Europe, Africa, and the Americas |
- The danger here is cascading tipping points, where one failure triggers others in a domino effect.
- 3. Extreme Heat and Uninhabitable Zones
- 1–3 billion people could live in regions as hot as the Sahara today.
- Widespread urban heat islands could cause mass mortality events in vulnerable cities.
- Poorer countries in the tropics (already low on resources) would bear the brunt.
- 4. Sea-Level Rise of 2+ Meters by 2100 (or faster)
- While 1–2 feet by 2100 is considered "likely," worst-case scenarios estimate 6–7 feet (2 meters or more) if ice sheet dynamics accelerate.
- Impacts:
- Submergence of major cities: New York, Miami, Mumbai, Jakarta, Dhaka, Shanghai.
- Massive displacement of coastal populations (hundreds of millions).
- Loss of freshwater aquifers and cropland from saltwater intrusion.
- 5. More Frequent & Stronger Extreme Weather
- Mega-hurricanes, floods, wildfires, and droughts become more destructive and common.
- What’s now a "100-year storm" might occur every 5–10 years.
- Costs of climate-related disasters could exceed $1–2 trillion annually by late-century.
- 6. Global Food System Collapse
- Simultaneous crop failures in multiple breadbasket regions (U.S. Midwest, China, India, Brazil).
- Rising temperatures and erratic rainfall patterns make agriculture highly unstable.
- Fish stocks collapse due to ocean warming and acidification.
- Results in global food shortages, price spikes, famines, and geopolitical tension.
- 7. Mass Migration and Conflict
- Climate stress could displace hundreds of millions, both internally and across borders.
- “Climate refugees” strain urban systems, public health, and political stability.
- Resource scarcity (especially water and food) may trigger conflict and war, particularly in fragile states.
- 8. Collapse of Natural Ecosystems
- Coral reefs vanish almost entirely beyond 2°C.
- Massive biodiversity loss on land and in oceans.
- Ecosystem services (pollination, clean water, carbon storage) are degraded.
- Forest diebacks reduce carbon uptake, accelerating warming.
- 9. Runaway Climate Feedbacks
- The scariest scenario: warming causes positive feedbacks that spiral out of control.
- Melting permafrost → methane release → more warming.
- Drought → forest fires → CO₂ spikes.
- Ice loss → less sunlight reflected → more heat absorbed.
- 10. Societal Collapse in Some Regions
- Combining environmental stress with political instability could lead to:
- State failure in vulnerable nations
- Rise of authoritarian regimes exploiting climate crises
- Widespread loss of life, cultural heritage, and governance systems
- In Summary: What Makes These Worst-Case Scenarios "Likely"?
- They are plausible but avoidable, depending on:
- Whether emissions peak and fall soon (e.g., under Paris Agreement targets)
- The presence (or absence) of global cooperation and climate adaptation
- Societal, economic, and political responses to compounding crises