Lecture 14 Climate Variability and Air Pollution

Human Influence and Climate Change

• GHGs and aerosols

• internal climate variability 

  • year to year variation in climate caused by internal climate processes

• human influence detectable when climate change signal is greater than climate variability

Climate Variability: El Nino Southern Oscillation

• main cause of year to year variability in global atmospheric weather patterns affecting climate over many regions

  • also slightly global mean temperatures

• El Nino and La Nina phases

• impacts on tropical and global climate

  • El Nino can cause flooding in tropical South America and dry conditions in South East Asia/Oceania

  • changes to the atmospheric circulation affect neighbouring regions hence ENSO impacts extend outside of tropics

El Nino

• the tropical eastern Pacific becomes unusually warm in winter/spring and this continues for about a year

• warm surface water propagates eastward

• surface pressure rises in the West and falls in the East

• the pressure gradient between East and West Pacific weakens hence the trade winds weaken

  • Southern Oscillation

• region of heaviest rainfall moves eastward

• drought in South East Asia, flooding in South America

La Nina

• colder than average SSTs in the tropical eastern Pacific

• higher than normal pressure in East Pacific, strong PGF, stronger trade winds

• low pressure and heavier than normal rainfall in South East Asia

• sinking air and drier weather over South America

• enhancement of normal conditions

Climate Variability: North Atlantic Oscillation

• atmosphere perturbation

• Positive Phase of Wintertime NAO

  • southerly flow from tropics leads to warmer weather in Europe

• Negative Phase of Wintertime NAO

  • cold air from Canada leads to colder weather in Europe

• varies year by year

NAO Positive Phase

• enhanced southerly flow from tropics keeps cold air in Canada

• jetstream and storm track in North Atlantic

• strong low pressure in high North Atlantic

• warmer and wetter in Northern Europe

• drier in Southern Europe and Northern Africa

• strong subtropical high pressure

NAO Negative Phase

• strong flow of cold air from Canada

• jetstream and storm track in mid-Atlantic

• weak low pressure in high North Atlantic

• colder and drier in Northern Europe

• wetter in Southern Europe and Northern Africa

• weak subtropical high pressure

Air Pollutants

• particulate matter (PM)

  • usually PM_2.5 referred to 

    • PM with aerodynamic diameter <2.5µm

  • solid or liquid

• PM_2.5 refers to size, composition varies

• either emitted as primary PM, or formed in atmosphere from gases as secondary PM

• Ozone (O3) 

• Nitrogen Dioxide (NO2)

• PM_2.5 and NO2 often co-emitted (traffic), as well as with CO2

• O3 and NO2 in urban environments usually inversely related

Types of Particulate Matter 2.5 (3, 4)

• inorganic (nitrate, sulphate, ammonium)

• organic (black carbon, organic carbon, dust, sea-salt)

• multiple anthropogenic and natural sources

Ozone

• major oxidant species in atmosphere

• secondary pollutant formed in the atmosphere in the presence of sunlight and precursor emissions

• greenhouse gas

Nitrogen Dioxide

• also a gaseous oxidant

• forms from nitrogen oxide (mainly anthropogenic sources)

The Case for Clean Air

• Ella Adoo-Kissi-Debrah 

• died in 2013, in 2020 Coroner’s court found air pollution ‘made a material contribution’ to her death

• first time ever that air pollution was listed as cause of death

• air pollution linked to asthma

• Ella exposed to excessive amount of NO2 from road traffic (south circular London)

Health Effects of Air Pollution

• short term

  • exacerbation of asthma

  • cough, wheezing and shortness of breath

• long term

  • stroke

  • lung cancer

  • respiratory conditions

  • cardiovascular disease

  • reduced life expectancy

• estimated that 28-36,000 deaths in UK per year linked to long-term exposure

Anthropogenic Sources of Air Pollution in UK

• energy industries

• manufacturing industries

• construction

• residential combustion

• road transport

• agriculture

• etc.

Natural Sources of Air Pollution

• smoke particles and gases from volcanoes and wildfires produce PM_2.5, PM₁₀, NO2 and O3

• methane emissions from wetlands, ruminants, decomposition of organic matter produce O3

• Biogenic Volatile Organic Compounds (BVOCs) from certain tree species produce PM_2.5 and O3

Air Pollution London 2019

• some areas of central London exceed UK annual objective for NO2

• large areas of central London exceed WHO guideline for PM_2.5 

WHO 2021 Guidelines

• challenging, especially for PM

• 5µgm^3 per year

• nowhere in London is below this annual limit for PM_2.5 

Anticyclones and Air Pollution

• increase air pollution

• high pressure leads to light winds and little atmospheric mixing

• reduced vertical dispersion

  • mostly descending air from above

  • temperature inversion at the top of the boundary layer limits vertical mixing

• no precipitation to aid ‘wet’ removal of PM

• strong sunshine allowing photochemical reactions, e.g. to form ozone

Future SSP Scenarios (6)

• global emissions of CO2 and of air pollutant emissions and precursors

  • near term climate forcers (NTCF)

• most scenarios show a reduction in global emissions, although there is large diversity between scenarios and regional variation

• future methane shows large variation between scenarios, important for ozone formation

• large PM_2.5 emission reductions in Asia with large emission reduction scenarios

• O3 reductions driven by large reductions in precursors (NOx and VOCs)

• SSP3-7.0 decrease in PM_2.5 in Europe and East Asia, increase in South Asia, Middle East, Nigeria and Uganda regions, increase in O3 across Asia, Africa, South America

Climate Change and Air

• climate change and air quality not necessarily hand in hand

• reduced sulphur emissions good for air quality, bad for climate change

• planting high BVOC trees good for sequestering carbon, bad for air quality

• biomass and hydrogen both increase NOx and PM_2.5