The greenhouse effect
and global warming
In this topic we will consider:
§ The natural greenhouse effect and greenhouse gases
§ The recent increase in greenhouse gases and the enhanced greenhouse effect.
§ Potential effects of global warming
§ Feedback mechanisms associated with global warming
§ Pollution management strategies to address global warming; global and local
§ Arguments surrounding global warming (for and against)
§ Conflicting perceptions about global warming/climate change
The greenhouse effect (6.1.1)
A garden greenhouse keeps plants warmer than they would be outside. It does this because the glass traps some of the Sun’s radiation energy. The atmosphere keeps the Earth warm in a similar way. This is because the energy entering our atmosphere is mostly short wave, which includes light energy and energy leaving the planet is mostly long wave, which includes heat energy. Some long wave radiation is absorbed by greenhouse gases before it escapes from the atmosphere and is reflected back to Earth. Without the greenhouse effect the Earth would be about 33°C cooler than today’s pleasant average of 15°C. The 'greenhouse effect' is a normal and necessary condition for life on Earth.
Look at the diagram on the front page. Neatly write the following labels onto the diagram in the correct places.
Natural greenhouse effect: http://sciencebitz.com/?page_id=582
Simulation: http://www.sumanasinc.com/webcontent/animations/content/greenhouse.html
P135 – ESS, Rutherford
If there was no atmosphere and the greenhouse effect did not exist, what would the normal temperature of the Earth be?
Approx -15 degrees celcius
Greenhouse gases
Historically, the main greenhouse gases were water vapour, carbon dioxide, nitrous oxide and methane.
Evidence for the link between the concentration of carbon dioxide in the atmosphere and global mean temperatures over the last quarter of a million years is shown in the graphs. Similar data exists going back hundreds of millions of years.
How was this data collected?
Ice cores in the Antarctic - Vostok
What conclusion can you draw from the two graphs?
There is a positive correlation between increased carbon dioxide levels and temperature levels, both temperature and carbon dioxide levels increased/fluctuated before human activity.
Reasons? – Milankovich cycles – changes in the orbit of the earth around the sun (elliptical), changes in solar radiation output (sunspots), changes in the axis tilt of the earth (from 23 half degrees), asteroids? Not really known but many theories?
James Lovelock Gaia hypothesis – plants regulate temp/atmosphere of earth and start negative feedback.
The effect of greenhouse gas increases is called the enhanced greenhouse effect or simply global warming.
Recent evidence for global warming
The following graph shows data collected at one of the most unpolluted places on Earth, Mauna Loa, Hawaii in the middle of the Pacific Ocean.
What is the increase in mean CO2 concentration in the last 40 years?
60ppm – from 310 to 370ppm
Look at the more detailed inset.
Why do CO2 levels decrease between May and September every year?
In the spring, leaves return to the trees and photosynthesis increases dramatically, drawing down the CO2in the atmosphere.
Why do the CO2 levels climb higher each year from October until May?
More organic matter is being decomposed by microbes releasing CO2 into the atmosphere (carbon cycle) e.g. leaf litter. During this decomposition, microbes respire and produce CO2, contributing to atmospheric CO2levels in the process. Thus over the course of the winter, there is a steady increase in CO2in the atmosphere.
Global mean temperatures since the Industrial Revolution
The graph below shows the Central England Temperatures, or CET data, which go back to 1660 when thermometers were first used.
The 20th century has clearly had the warmest in recent history. In the last 100 years the average temperature has increased by 0.6°C, but this warming has speeded up, jumping 0.5°C in the last 25 years. The 10 hottest years since records began have been since 1990. Computer modelling is now used to predict future temperature increases, which currently are between 1.4 – 5.8°C.
What evidence is there that methane, nitrous oxide and CFC-11 may also be contributing to global warming? Look at the graph on p137 of ESS Rutherford.
What do we mean by the term ‘’Radiative forcing’’?
This means the change in the balance of radiation coming into the atmosphere and radiation going out of the atmosphere. If positive earth’s surface warms up, if negative it is cooled.
Greenhouse gases would be positive – the biggest being CO2.
What does the term albedo mean? Why is it so important?
Albedo is the % reflectivity of light from a surface - the proportion of the light or radiation that is reflected by a surface, high albedo is lighter surfaces e.g snow/ice, low albedo is darker surfaces e.g. tarmac/concrete.Anthropogenic greenhouse gas emissions (6.1.2)
The list of greenhouse gases now includes carbon dioxide, water vapour, nitrous oxide, methane and chlorofluorocarbons (CFCs). Complete the table from the information on p137 of ESS Rutherford:
Greenhouse gas / Pre-industrialConcentration
ppm / Present concentration / % green-house
effect / Atmospheric life times / years
CO2 / 270 / 379 / 50-60 / 50-200
Methane, CH4 / 0.7 / 1.7 / 20 / 12
Nitrous oxide / 0.27 / 0.31 / 4-6 / 140
CFC-11 / 0 / 0.00025 / 14 / 45
Why is water vapour seldom mentioned as a greenhouse gas?
It occurs naturally in the atmosphere so is part of the natural greenhouse effect. It is difficult to measure/quantify the human input.
Anthropogenic sources of greenhouse gases
Complete the table with sources due to human activities.
Greenhouse gas
/ Sources due to human activitiesCarbon dioxide
Methane
Ozone
Nitrous Oxide
CFC’s (chloroflurocarbons)
Water vapour
Use your Rutherford book (p133 onwards) and the information pack to complete these questions.
a) Approximately what proportion of world greenhouse gas emissions come from energy?
61.4% of the world's greenhouse gas emissions come from energy.
b) Which part of the energy sector emits most greenhouse gases (by volume)?
Electricity and Heat are the energy sectors that emit the most greenhouse gases
c) What are the main greenhouse gases emitted? What is their relative % contribution in world emissions?
The main greenhouse gases emitted are carbon dioxide (60%), methane (20%) and nitrous oxide (6%).
d) Apart from the energy sector, what are the main sources of greenhouse gases?
Other than the energy sector, the main sources of greenhouse gases are land use and agriculture.
e) What are the main sources of
(i) methane?
The main sources of methane are from agriculture (cows farting and rice fields) and industrial processes.
(ii) nitrous oxides?
The main sources of nitrous oxides are from agriculture