6. Our Changing Planet Week 6

Lectures 11 and 12

What are the three components of the Triangle Risk model for natural disasters?

Hazard (what the disaster is and how powerful it is), Vulnerability (population size, poverty levels, recovery capacity, warning systems), Exposure (where people are, whether they are protected, whether hazard impact has been reduced)

Why are so-called "natural" disasters considered human disasters?

Because disasters occur due to where people live and how they react to changes in their environment — the same hazard without people present does not constitute a disaster

Why has the number of reported natural disasters increased, despite no increase in geological events?

Because climatological risks such as wildfires, extreme temperatures, and droughts have increased, raising the hazard level

By how much has death from natural disasters decreased over time, despite population growth?

Death from natural disasters has decreased approximately 10-fold, even as global population grew from 2 billion to 8 billion

What event marked the beginning of earthquake science, and what happened?

The 1755 Lisbon Tsunami, which devastated Lisbon and allowed scientists to model how seismic waves spread, including effects on the south coast of England

How does water depth affect a tsunami wave?

Speed is determined by water depth — waves travel very fast in deep open ocean but slow in shallow water, causing wave amplitude to increase as it approaches the coast

What typically causes fatalities in a tsunami, rather than the water itself?

Debris picked up by the wave, such as trees, cars, and rubble

How many people died in the 2004 Indonesian tsunami, and where did it occur?

Approximately 250,000 people died, originating at the Indian Ocean East boundary

How did Japan's 2011 tsunami compare to the 2004 Indonesian tsunami, and what reduced its death toll?

It was a similar or larger size, but Japan's extensive tsunami defences reduced deaths to approximately 25,000

Why do tsunamis disproportionately kill the very young and very old?

Because they are more physically fragile and less able to survive the physical trauma of being struck by debris carried by the wave

How long does a tsunami originating near Hawaii take to reach California and Chile?

Approximately 6 hours to California and 15 hours to Chile

What is an avalanche, and what engineering measures are used to manage them?

A rapid flow of a mass of snow down a slope, managed using snow bridges, avalanche sheds, splitting wedges, deflecting walls, and avalanche breakers

What is a tornado, and how does it form?

A huge rotating column of air that forms when a storm system develops enough spin from atmospheric rotation to extend from the storm cloud down to the ground

What is Tornado Alley, and why does it exist?

A region in central USA where warm and cold air masses meet, generating large storms with sufficient atmospheric spin to produce tornadoes

What is the maximum ground width of a tornado's damage path?

Up to approximately one mile wide

What are the Fujita scale categories and their associated wind speeds?

F0 (

What are hurricanes, typhoons, and cyclones, and how do they differ?

They are the same type of massive extra-tropical storm, called hurricanes in America, typhoons in Southeast Asia, and cyclones in the Indian Ocean

What sea surface temperature is required to form a hurricane, and why?

At least 26°C, as this generates sufficient heat and evaporation to drive the spinning storm system upwards from the ocean surface

How do hurricanes differ from tornadoes in terms of size and origin?

Hurricanes originate over the ocean and spin upwards, and can be the size of France, whereas tornadoes spin downwards over land with a maximum width of about one mile

What does the Saffir-Simpson scale measure, and what is its key limitation?

It measures hurricane wind damage only and ignores rainfall and flooding, which can cause far greater casualties

What is an example of the Saffir-Simpson scale underestimating a hurricane's danger?

Hurricane Mitch in 1998 was only a Category 3, yet killed over 20,000 people and made 2.5 million homeless due to catastrophic flooding

What non-linear cost relationship is seen in the Saffir-Simpson scale?

Damage costs are not proportional to category — the largest jump in total economic damage occurs between Category 2 and Category 3

What are the three stages of disaster cycle management?

Stage 1: assess damage and provide immediate rescue and medical aid, Stage 2: provide food, water, shelter, and prevent disease, Stage 3: restore communications and economic activity, then rebuild more resiliently

Why do disease deaths after a natural disaster often equal the initial death toll?

Because disrupted sanitation, contaminated water, and lack of medical care create conditions in which infectious disease spreads rapidly among survivors

What is the disaster management 'rule of three'?

3 minutes without air (critical for avalanche/mudslide burial), 3 days without water (for those trapped in rubble), 3 weeks without food

How has Bangladesh reduced cyclone deaths over time?

Deaths fell from ~300,000 in the 1970s to ~3,000 by 2007 through investment in meteorological forecasting, a low-tech radio and bicycle warning network, raised flood shelters, clean water access, and floating agriculture

In the Bangladesh case study, which component of the risk triangle was reduced to lower disaster impact?

Vulnerability was reduced, while exposure and hazard remained the same

How could climate change and population growth worsen Bangladesh's future disaster risk?

Climate change increases the hazard (e.g. more intense cyclones), while population growth increases both vulnerability and exposure

What specific threat does Bangladesh face from sea level rise by 2100?

A sea level rise of around 1 m combined with an additional ~50 cm of relative sinking (due to reduced sediment deposition from upstream dams) could place 20% of the country underwater, with further areas suffering salt intrusion

Why was New Orleans described as a particularly poorly chosen location for a city?

It sits near the coast with the Mississippi River on one side and a lake on the other, with the majority of the city below sea level and in a region frequently hit by hurricanes

What did the 2003 FEMA Hurricane Pam exercise predict, and what action was taken?

It simulated a Category 5 hurricane hitting New Orleans and predicted levee failure, but no significant preparatory action was taken

What proportion of New Orleans residents could not evacuate ahead of Hurricane Katrina, and why?

Approximately 127,000 people had no access to a car, and evacuating even 30,000 people by bus would have taken 10 days

What was the key structural failure of New Orleans's levees during Hurricane Katrina?

The levees were only designed to withstand a Category 3 hurricane, were 14.5 feet high (sea level rose to 10.6 feet, so they were not overtopped), and were not interlocked, so water pressure caused them to collapse sideways

What were the human and financial costs of Hurricane Katrina?

1,836 dead, millions homeless, $10.5 billion in federal aid, a $60 billion insurance claim reduced to $30 billion in court (as damage was ruled flood rather than hurricane-related), and $81 billion in total property damage — preventable for $2 billion in levee upgrades

Why was the legal ruling on Hurricane Katrina insurance claims particularly damaging for victims?

Because poor residents had hurricane insurance but not flood insurance, and a court ruled that the damage was caused by flooding from broken levees rather than directly by the hurricane wind

What was the impact of Hurricane Mitch (1998) and three successive cyclones hitting Mozambique in 2000?

Hurricane Mitch killed 20,000 people and made 2.5 million homeless, setting back Central America's economy by about ten years, Mozambique's economy was similarly set back by ten years despite it being the fastest-growing economy in Africa at the time

What is the primary driver of anthropogenic climate change?

Greenhouse gas (GHG) emissions from human activity, with energy production and use being the largest single contributor

How do greenhouse gases cause warming?

They trap heat by reflecting some of the Sun's energy back to Earth's surface rather than allowing it to escape into space, and increasing GHG concentrations increase the amount of heat trapped

What proportion of human-caused warming since the 1990s is attributable to CO₂?

CO₂ is responsible for approximately 80% of the influence on temperatures since the 1990s

How does recent CO₂ increase compare to natural historical rates?

A 70 ppm increase occurred in just 30 years, roughly equivalent to the CO₂ increase across 6,000 years of deglaciation

What were global fossil fuel CO₂ emissions in 2024, and what is projected for 2025?

37.8 ± 2 GtCO₂ in 2024, with projections of 38.1 ± 2 GtCO₂ for 2025, approximately 69% above 1990 levels

Which global event caused the largest single-year decrease in CO₂ emissions?

The COVID-19 pandemic, which reduced CO₂ emissions by approximately 2 gigatonnes

What is methane's (CH₄) relative contribution to anthropogenic warming, and how potent is it?

Responsible for approximately 30% of global warming since the industrial revolution, with a 12-year atmospheric lifespan and a heat-trapping capacity about 80 times greater than CO₂ over a 20-year period

What are the main human sources of methane emissions?

Coal mining, oil and gas extraction, landfills and waste, and agricultural sources including livestock fermentation and rice cultivation

What is nitrous oxide's (N₂O) relative contribution to warming, and how potent is it compared to CO₂?

Responsible for approximately 10% of global warming since the industrial revolution, and approximately 270 times more potent than CO₂

What is N₂O's significance beyond warming?

It is also the top ozone-depleting substance currently being released into the atmosphere

What triggered the sharp increase in N₂O emissions from around 1950?

The widespread introduction of synthetic nitrogen fertilisers in agriculture, linked to the development of the Haber-Bosch process

What warning did the Global Nitrous Oxide Assessment of 2024 issue?

Without urgent action on N₂O, rising levels alone could push global temperatures past the 1.5°C limit, potentially making CO₂ reduction efforts insufficient to meet Paris Agreement targets

According to the IPCC, what is the likely total human-caused global surface temperature increase from 1850–1900 to 2010–2019?

Approximately 0.8°C to 1.3°C, with a best estimate of around 1.07°C

How does regional warming compare to the global average?

The Arctic is warming approximately 4 times faster than the global average, and Europe is warming approximately 1°C above the global average

What are Shared Socioeconomic Pathways (SSPs)?

Scenarios defined in the IPCC 6th Assessment Report (2021) that model how socioeconomic factors might evolve to influence future GHG emissions and climate change

What does SSP1-1.9 require to limit warming to 1.5°C by 2100?

Net-zero CO₂ emissions by 2050 and massive carbon removal

What does SSP5-8.5 (business-as-usual) project for CO₂ and temperature?

A doubling of CO₂ by 2050 and more than 4°C of warming

What is a climate tipping point, and what are some examples?

Thresholds beyond which changes become self-reinforcing and potentially irreversible, including Arctic winter sea ice loss, Amazon rainforest dieback, AMOC collapse, Greenland and Antarctic Ice Sheet collapse, and boreal permafrost thaw

What is the IPCC, and what is its role?

The Intergovernmental Panel on Climate Change, a UN body founded in 1988 in Geneva that reviews all relevant scientific literature to provide governments with scientific information for climate policy, without conducting its own original research

What is the Conference of the Parties (COP), and how many parties are involved?

The annual international meeting of the United Nations Framework Convention on Climate Change (UNFCCC), involving 198 parties (197 states plus the EU), aimed at stabilising GHG concentrations at safe levels

What did the Paris Agreement (2015) commit to, and what emission reductions does it require?

A legally binding international treaty to limit warming to below 2°C and preferably 1.5°C above pre-industrial levels, requiring a 45% reduction in emissions by 2030 and net zero by 2050

As of June 2024, how many countries have adopted net-zero pledges, and what proportion of global emissions do they represent?

107 countries responsible for approximately 82% of global greenhouse gas emissions

What was notable about the COP30 2025 outcome regarding fossil fuels?

The words "fossil fuels" were absent from the final text, despite being central to both the Glasgow Climate Pact (2021) and the UAE consensus (2023), though a proposed roadmap called the Global Mutirao text acknowledged overshoot of the Paris Agreement for the first time

What is net zero, and how is it defined in the context of the Paris Agreement?

Cutting carbon emissions to the level that can be absorbed and stored by natural and technological carbon removal measures, leaving a net balance of zero additional CO₂ in the atmosphere

What does the likelihood table for limiting warming below specific temperature thresholds show about current policies?

Under current policies continuing, there is 0% chance of staying below 1.5°C, 8% chance of staying below 2°C, and 80% chance of staying below 3°C — only with conditional NDCs plus all net-zero pledges does the probability of staying below 1.5°C rise to 21