3. Impacts of Climate Change on Ecosystems and People

Climate change is not a future problem — its effects are already observable across ecosystems and human societies.

Ecological impacts:

• Habitat shifts: as temperatures warm, species move poleward and to higher elevations to track suitable climate. Species that cannot move fast enough face extinction. Mountain-top species have nowhere to go.
• Phenological mismatches: timing of seasonal events (when flowers bloom, insects hatch, birds migrate) is shifting. If a bird arrives on its breeding grounds before the insect hatches it depends on, it cannot feed its chicks — a phenological mismatch.
• Coral bleaching: corals have a symbiotic relationship with algae (zooxanthellae) that live in their tissues. Warming water (even 1–2°C above normal, sustained for weeks) causes corals to expel the algae, turning white ("bleaching"). Without algae, corals starve. Repeated bleaching events kill coral reefs. The Great Barrier Reef has experienced severe bleaching in recent years.
• Ocean acidification: rising ocean acidity dissolves the calcium carbonate shells and skeletons of oysters, mussels, pteropods (sea snails), and corals, threatening marine food webs.
• Sea level rise and coastal ecosystems: salt marshes and mangroves — critical nurseries for fish and buffers against storms — are being drowned by rising seas.
• Increased wildfire: warming and drying conditions increase wildfire frequency and intensity, affecting forests worldwide.

Human / social impacts:

• Food security: changing rainfall and temperature patterns shift where crops can grow. Extreme heat reduces crop yields. More frequent droughts threaten agriculture in already-stressed regions.
• Water availability: many regions depend on glacial meltwater for fresh water during dry seasons. As glaciers disappear, summer water supplies decline.
• Sea level rise and flooding: coastal communities face increased flooding and storm surge risk. Low-lying island nations (Tuvalu, Maldives) face existential threat.
• Human health: heat waves, expanded range of mosquito-borne diseases (malaria, dengue, Zika), air quality degradation from wildfires and heat-driven ozone.
• Climate refugees and conflict: resource competition and displacement of people from areas made uninhabitable by drought or flooding increase political instability.