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Four basic facts about climate change that are not disputed:

1. Global temperatures have increased over the last century

This slide shows increases in globally averaged temperature changes from 1860 (when

accurate temperature records began) to 2005. It shows that global temperatures are

nearly 1deg C warmer than they were 150 years ago. Crucially, temperatures are now

changing at an unprecedented rate, and the 9 of the 10 warmest years ever recorded

have all occurred since 2001. This warming is having a number of consequences which

the following slides will highlight.

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Fact 2 there has been a measured rise in CO2 in the atmosphere since 1850 –

more than 500, 000 million tonnes

The carbon dioxide content of the atmosphere is gradually and steadily increasing. The

graph shows the CO2 concentration at the summit of Mauna Loa in Hawaii from 1958

through 1999. The values are in parts per million (ppm). The seasonal fluctuation is

caused by the increased uptake of CO2 by plants in the summer. (By November 2010, its

concentration had risen to 389 ppm.)

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Fact 3 Humans have contributed significantly to this rise since the industrial

revolution >1 million million tonnes

The increase in CO2 probably began with the start of the industrial revolution. Samples

of air trapped over the centuries in the glacial ice of Greenland show no change in CO2

content until 300 years ago. Since measurements of atmospheric CO2 began late in the

nineteenth century, its concentration has risen over 20%. This increase is surely

"anthropogenic"; that is, caused by human activities: burning fossil fuels (coal, oil,

natural gas) which returns to the atmosphere carbon that has been locked within the

earth for millions of years.

clearing and burning of forests, especially in the tropics.

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Fact 4 CO2 has a property that enables it to trap heat from light radiated off

the earth in the outer atmosphere (co2 absorbs more heat from reradiated

light than air does)

Carbon dioxide traps infrared radiation (commonly known as thermal radiation).

This has been proven by laboratory experiments and satellites which find less

heat escaping out to space over the last few decades (see ). This is direct

evidence that more CO is causing warming.

The past also tells an interesting story. Ice cores show that in the Earth’s past, CO

went up temperature initially increased. This “CO lag” means temperature

affects the amount of CO in the air. So warming causes more CO and more CO

causes extra warming. Put these two together and you get positive

feedback. Positive or negative feedback don’t necessarily mean good or bad.

Positive feedbacks strengthen any climate change already underway

while negative feedbacks suppress (weaken) any climate change. In the past

when climate warmed due to changes in the Earth’s orbit, this caused the ocean

to release more CO into the atmosphere resulting in the following effects:

• The extra CO in the atmosphere amplified the original warming. That’s the

positive feedback.

• The extra CO mixed through the atmosphere, spreading greenhouse warming

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across the

globe.

The ice core record is entirely consistent with the warming effect of CO . In fact,

the dramatic warming as the planet comes out of an ice age cannot be

explained without the feedback from CO . The CO lag doesn’t disprove the

warming effect of CO . On the contrary, it provides evidence of a positive climate

feedback.

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Causal linkage 1. Humans cause the rise in CO2

Causal linkage 2 . Increased CO2 leads to increased temperature

Scientists have done lots of research on the energy we get from the Sun and have

been able to rule that out as the main cause. Lots of natural cycles have been

identified in the climate, such as El Niño, but none of the one's we know about

could cause the relatively big, long-term changes we've seen.

Therefore, there's overwhelming and growing evidence that the warming we've

seen is due to increasing amounts of greenhouse gases in the atmosphere. It's

very likely this warming has been caused by human activity, such as burning fossil

fuels (like petrol and coal) and changing land use (such as chopping down forests

for cattle grazing).

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How do we know that the warming we have seen over the past few decades is caused

by human activities?

Global climate is influenced by a number of factors including natural cycles in the

climate, volcanic eruptions and by fluctuations in the amount of energy we receive from

the sun. However, none of these factors is sufficient on its own to cause changes on the

scale that have been observed over the past century.

This slide shows results of experiments conducted using global climate models run to

simulate the recent changes in climate (ie the red line on the graph) by considering only

natural changes in climate (green). You can see from this image that it is not possible to

accurately simulate changes in global temperature since 1950 if only natural changes in

climate only are taken into account. Observed changes are warmer than simulations.

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When changes in human activities (ie. increased emissions of greenhouse gases such as carbon

dioxide and methane) are included, the model simulation fits the observed temperature record

very well.

In other words, we can only replicate the warming we have seen in recent years if human

emissions of greenhouse gases are taken into account.

Measurements of the type of carbon found in the atmosphere show that fossil

fuel burning is dramatically increasing levels of carbon dioxide

(CO ) in the atmosphere. Satellite and surface measurements find that extra CO is

trapping heat that would otherwise escape out to space. There

are a number of warming patterns consistent with an increased greenhouse

effect. The whole structure of our atmosphere is changing.

The evidence for human caused global warming is not just based on theory or

computer models but on many independent, direct observations

made in the real world

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About 2500 scientists are officially on the IPCC. But their reports are simply a summary of the work of many more, from all around the world. They have thousands of references to the scientific literature.

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So, how much warmer could it get?

This slide shows how global temperature would rise according to four possible scenarios

of human activity and development. The black line shows how much warmer

it would get if we carry on emitting greenhouse gases as we are today.

The green lines shows how much it could warm if everyone adopts

more climate friendly ways of working and living.

There are 2 important points to take from this graph.

It is important to be aware that projections from climate models are

always subject to uncertainty because of limitations on our knowledge

of how the climate system works and on the computing resources

available. Different climate models can give different projections.

The projections are also based on emissions scenarios, such as the

level of CO2 emissions increasing or decreasing. Many different

scenarios are used, based on estimates of economic and social growth,

and this is one of the major sources of uncertainty in climate

prediction. But even if greenhouse gas emissions are substantially

reduced, the long lifespan of CO2 in the atmosphere means that we

cannot avoid further climate change due to CO2 already in the

atmosphere.

Despite the uncertainties, all models show that the Earth will warm in

the next century, with a consistent geographical pattern.

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Having said that global warming is not just part of a natural cycle, there will always be

natural variability in the climate – they are two different things. Human memory is short

term e.g. snow in February 2009, doesn’t square with a lot of peoples concept of global

warming and climate change. Similarly, people can always make reference to a year

when it was warmer e.g. 1976, and this might be another obstacle to persuading people.

Observations and model runs for ppt. anomaly, shows that there is large variation in this,

and might help to explain why it is difficult for people to experience climate change (in

the way that they view it) in their lifetime, e.g. Climate change does not mean that there

will never be snow again, just that the incidence of such events in the UK will decrease,

as has been the case. A problem of perception of what climate change is, perhaps the

effect of the media, and the human memory/experience. Global warming/Climate

change is a trend not a single weather event.

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Evidence shows rainfall patterns are changing across the globe. Generally, wet places are

becoming wetter and dry areas are becoming drier. However, there are also changes

between seasons in different regions. For example, rainfall in the UK during summer is

decreasing, while in winter it is increasing. In the UK, the growing season has lengthened

due to Spring starting earlier and the delayed onset of autumn/winter. Wildlife experts

have noted that many species are changing their behaviour, from butterflies appearing

earlier in the year to birds starting to change their migration patterns.

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Long-term drought could cause huge migration and subsequent security issues

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