5. Our Changing Planet Week 5

Lecture 9 and 10

What are the four phases of human impact on the biosphere?

Phase 1: human dispersal ~100,000 years ago

Phase 2: agriculture ~10,000 years ago

Phase 3: Columbian Exchange ~600 years ago

Phase 4: Industrial Revolution ~60-70 years ago

What is the estimated upper limit of Earth's human carrying capacity?

13–15 billion people

What is the definition of megafauna?

Animals weighing more than 44 kg

How many genera of megafauna existed 50,000 years ago, and how many had gone extinct by 10,000 years BP?

More than 150 genera existed

at least 97 had become extinct by 10,000 years BP

BY 2018 CE, what percentage of wild mammals remained compared to before human civilisation?

83%

What proportion of global mammal biomass are wild animals ?

3%

How has the number of trees on Earth changed since the dawn of agriculture?

Decreased from 6 trillion to 2 trillion trees

What are the top five crops produced by farmland annually?

Sugar cane, maize, rice, wheat, and potatoes

What is the Columbian Exchange?

The movement of crops, animals, and other species between continents, mainly from South America to Europe, as humans became a global species

What effect did the introduction of the potato have on human populations?

It led to a 25–26% increase in total population and a 27–34% increase in urbanisation

What is sugarcane's global significance?

It is the world's largest crop by production

• contributes ~70% of world sugar production

• potential for ethanol-based biofuel production

How much of Mauritius's arable land does sugarcane occupy?

85% of all arable land and 40% of the whole island

What percentage of Mauritius's native forest cover remains today?

Approximately 2% of high-quality native forest cover

Why are island species particularly vulnerable to human impacts?

often have high proportions of endemic species - traits like flightlessness and gigantism

→especially susceptible to habitat loss + invasive species

What percentage of all bird extinctions have occurred on islands, despite islands being what fraction of Earth's area?

Close to 90% of bird species extinctions, despite islands being only 3.5% of Earth's area

What became the leading driver of tropical forest loss in 2024 for the first time?

Fire

What is the projected extinction rate for species under current trajectories of habitat loss and warming?

Nearly 40% of all species could face extinction by the end of this century

What percentage of terrestrial biodiversity is found in forests, and how much CO₂ do they absorb?

80% of terrestrial biodiversity

forests absorb approximately 2.6 billion tonnes of CO₂ annually

What percentage of global food crops rely on pollinators, and what is their annual contribution to agricultural output?

75% of global food crops rely on pollinators, contributing US$235–577 billion annually

What is the current biodiversity intactness figure due to land use effects?

Diversity has fallen to 84.6%

What caused the first major mass extinction (~440 mya)?

Sudden large-scale glaciation (Ordovician mass extinction)

25% of families lost

What caused the second major mass extinction (~370 mya)?

Volcanic activity or ocean anoxia (Late Devonian)

19% of families lost

What caused the third major mass extinction (~252 mya) and how severe was it?

Volcanic activity (end of Permian)

the greatest mass extinction event, losing 54% of families

What caused the fourth major mass extinction (~201 mya)?

A volcanic event (end of Triassic)

What caused the fifth major mass extinction (~65 mya)?

A meteor impact (end of Cretaceous)

17% of families lost, including the non-avian dinosaurs

What was the Bramble Cay Melomys and why is it significant?

The first mammal confirmed extinct due to human-caused climate change

rising sea levels destroyed its vegetation, and it was declared extinct by 2019

At what rate are species currently going extinct compared to pre-human baselines?

Approximately 1000 times the background extinction rate

What percentage of terrestrial land has been affected by human activity?

Over 77%

What are anthromes?

Anthropogenic biomes — areas whose character has been fundamentally altered by human land use

What contribution do invasive alien species make to extinctions and economic damage?

• contribute to 60% of species extinctions

• cause approximately US$423 billion in global economic damage annually

What is biomagnification?

The process by which toxic pollutants accumulate and increase in concentration as they move up the food chain, reaching lethal levels in top predators

What were the ecological effects of the insecticide DDT?

eggshell thinning of birds

mass population declines in bird species

toxicity to aquatic organisms

neurotoxicity in animals and humans

How is anthropogenic climate change affecting the water cycle?

It is intensifying the water cycle, causing

1. more intense rainfall

2. shorter ice persistence

3. decreased groundwater recharge

4. ocean heating

How is Arctic precipitation expected to change by 2100?

It is projected to increase by 20–60%, shifting from snow-dominated to rain-dominated precipitation

What is the Clausius-Clapeyron relationship in the context of precipitation?

As temperature rises due to GHG emissions → atmospheric humidity increases → intensifying extreme precipitation

— humidity is expected to increase by 7% per 1 K rise in temperature

How are Asian monsoons expected to change under climate change?

They will become hotter, wetter, and more unpredictable

individual storm rainfall could rise by up to 28%

2–3°C above pre-industrial levels

What sea level rise is projected by 2100 due to accelerated ice sheet melt?

Between 0.28 m and 1.01 m

complete melting of all ice sheets could cause ~60 m of sea level rise

What is AMOC and what is the concern regarding freshwater input?

The Atlantic Meridional Overturning Circulation

increased freshwater input from melting ice could cause its collapse, disrupting heat transfer around the ocean and atmospheric circulation

What has happened to wetlands and freshwater wildlife populations since 1970?

35% of wetlands have been lost

monitored freshwater wildlife populations have declined by an average of 85%

What is eutrophication and what causes it?

The enrichment of water bodies with excess nitrogen and phosphorus (often from agricultural runoff), causing algal blooms, reduced light penetration, oxygen depletion, and loss of aquatic biodiversity

What are diatoms, and how are they used as a proxy for eutrophication?

Diatoms: unicellular algae preserved in lake sediments

changes in diatom species assemblages over time indicate past changes in nutrient levels and water quality

What are the two sources of microplastics in the environment?

Those manufactured to be microplastics, and those resulting from the degradation of larger plastic items into particles below 5 mm

What proportion of plastic waste entering the ocean each year comes from rivers?

An estimated 1.15–2.41 million tonnes per year, with ~80% distributed by over 1,000 rivers, predominantly small rivers in densely populated urban areas

Why do microplastic concentrations in rivers peak between May and October?

Due to the monsoon season — increased rainfall and surface runoff from urban areas raises the velocity and volume of plastics entering waterways

How does seasonal lake stratification affect methane emissions?

When stratified, anoxic bottom waters produce methane

when the lake mixes during spring and autumn turnover, accumulated methane is released into the atmosphere

What is the Land to Ocean Aquatic Continuum (LOAC)?

The pathway by which carbon is transported from terrestrial environments through freshwater systems (rivers, lakes, wetlands) to the ocean, now recognised as a key component of the global carbon cycle

How acidic are ocean surface waters compared to pre-industrial levels?

Approximately 30% more acidic than pre-industrial levels, due to absorption of atmospheric CO₂

How has the rate of sea surface temperature increase changed over recent decades?

From 0.06°C per decade in the 1980s → 0.27°C per decade currently

How does sea ice loss contribute to arctic amplification?

Sea ice reflects solar radiation (high albedo)

melting exposes darker ocean water with lower albedo, which absorbs more heat, further raising temperatures in a positive feedback cycle

How has climate change affected Atlantic hurricane intensity?

• warming of Atlantic sea surface temperatures has doubled the probability of extremely active tropical cyclone seasons

• intensified the max wind speeds of ~80% of Atlantic Basin hurricanes from 2019–2023 by an average of 18 mph

What are planetary boundaries?

Thresholds within which humanity can safely operate without causing irreversible damage to Earth systems

How many planetary boundaries were crossed in 2009, 2015, 2023, and 2025?

3 crossed in 2009, 4 in 2015, 6 in 2023, 7 in 2025

What is ecological overshoot?

When humanity's demand for natural resources and ecosystem services exceeds Earth's ability to regenerate them within one year

How long would it take key biogeochemical cycles to recover if human impacts stopped today?

Carbon cycle: ~200,000 years

Phosphorus cycle: ~20,000 years

Nitrogen cycle: less than 10,000 years

What is habitat connectivity and why does it matter for biodiversity?

The degree to which habitats are linked, allowing species to move between them

it is the main cause of species decline, with threats from land use reaching up to 90% in highly fragmented landscapes

What is the annual volume of fish removed from the ocean through exploitation?

50 million tonnes removed annually, with a further 80 million tonnes farmed

What are Organic Carbon Accumulation Rates (OCAR) and what factors influence them?

The rate at which organic carbon is buried in lake sediments

rates are higher in the Holocene than the Pleistocene, higher in the tropics, and higher in warm and dry regions than wetter ones