Four basic facts about climate change that are not disputed:1. Global temperatures have increased over the last centuryThis slide shows increases in globally averaged temperature changes from 1860 (whenaccurate temperature records began) to 2005. It shows that global temperatures arenearly 1deg C warmer than they were 150 years ago. Crucially, temperatures are nowchanging at an unprecedented rate, and the 9 of the 10 warmest years ever recordedhave all occurred since 2001. This warming is having a number of consequences whichthe following slides will highlight.2
Fact 2 there has been a measured rise in CO2 in the atmosphere since 1850 –more than 500, 000 million tonnesThe carbon dioxide content of the atmosphere is gradually and steadily increasing. Thegraph shows the CO2 concentration at the summit of Mauna Loa in Hawaii from 1958through 1999. The values are in parts per million (ppm). The seasonal fluctuation iscaused by the increased uptake of CO2 by plants in the summer. (By November 2010, itsconcentration had risen to 389 ppm.)3
Fact 3 Humans have contributed significantly to this rise since the industrialrevolution >1 million million tonnesThe increase in CO2 probably began with the start of the industrial revolution. Samplesof air trapped over the centuries in the glacial ice of Greenland show no change in CO2content until 300 years ago. Since measurements of atmospheric CO2 began late in thenineteenth 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 theearth for millions of years.clearing and burning of forests, especially in the tropics.4
Fact 4 CO2 has a property that enables it to trap heat from light radiated offthe earth in the outer atmosphere (co2 absorbs more heat from reradiatedlight than air does)Carbon dioxide traps infrared radiation (commonly known as thermal radiation).This has been proven by laboratory experiments and satellites which find lessheat escaping out to space over the last few decades (see ). This is directevidence that more CO is causing warming.The past also tells an interesting story. Ice cores show that in the Earth’s past, COwent up temperature initially increased. This “CO lag” means temperatureaffects the amount of CO in the air. So warming causes more CO and more COcauses extra warming. Put these two together and you get positivefeedback. Positive or negative feedback don’t necessarily mean good or bad.Positive feedbacks strengthen any climate change already underwaywhile negative feedbacks suppress (weaken) any climate change. In the pastwhen climate warmed due to changes in the Earth’s orbit, this caused the oceanto release more CO into the atmosphere resulting in the following effects:• The extra CO in the atmosphere amplified the original warming. That’s thepositive feedback.• The extra CO mixed through the atmosphere, spreading greenhouse warming5
across theglobe.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 beexplained without the feedback from CO . The CO lag doesn’t disprove thewarming effect of CO . On the contrary, it provides evidence of a positive climatefeedback.5
Causal linkage 1. Humans cause the rise in CO2Causal linkage 2 . Increased CO2 leads to increased temperatureScientists have done lots of research on the energy we get from the Sun and havebeen able to rule that out as the main cause. Lots of natural cycles have beenidentified in the climate, such as El Niño, but none of the ones we know aboutcould cause the relatively big, long-term changes weve seen.Therefore, theres overwhelming and growing evidence that the warming weveseen is due to increasing amounts of greenhouse gases in the atmosphere. Itsvery likely this warming has been caused by human activity, such as burning fossilfuels (like petrol and coal) and changing land use (such as chopping down forestsfor cattle grazing).6
7How do we know that the warming we have seen over the past few decades is causedby human activities?Global climate is influenced by a number of factors including natural cycles in theclimate, volcanic eruptions and by fluctuations in the amount of energy we receive fromthe sun. However, none of these factors is sufficient on its own to cause changes on thescale that have been observed over the past century.This slide shows results of experiments conducted using global climate models run tosimulate the recent changes in climate (ie the red line on the graph) by considering onlynatural changes in climate (green). You can see from this image that it is not possible toaccurately simulate changes in global temperature since 1950 if only natural changes inclimate only are taken into account. Observed changes are warmer than simulations.
8When changes in human activities (ie. increased emissions of greenhouse gases such as carbondioxide and methane) are included, the model simulation fits the observed temperature recordvery well.In other words, we can only replicate the warming we have seen in recent years if humanemissions of greenhouse gases are taken into account.
Measurements of the type of carbon found in the atmosphere show that fossilfuel burning is dramatically increasing levels of carbon dioxide(CO ) in the atmosphere. Satellite and surface measurements find that extra CO istrapping heat that would otherwise escape out to space. Thereare a number of warming patterns consistent with an increased greenhouseeffect. The whole structure of our atmosphere is changing.The evidence for human caused global warming is not just based on theory orcomputer models but on many independent, direct observationsmade in the real world9
About 2500 scientists are officially on the IPCC. But their reports are simply asummary of the work of many more, from all around the world. They havethousands of references to the scientific literature.10
11So, how much warmer could it get?This slide shows how global temperature would rise according to four possible scenariosof human activity and development. The black line shows how much warmerit would get if we carry on emitting greenhouse gases as we are today.The green lines shows how much it could warm if everyone adoptsmore 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 arealways subject to uncertainty because of limitations on our knowledgeof how the climate system works and on the computing resourcesavailable. Different climate models can give different projections.The projections are also based on emissions scenarios, such as thelevel of CO2 emissions increasing or decreasing. Many differentscenarios are used, based on estimates of economic and social growth,and this is one of the major sources of uncertainty in climateprediction. But even if greenhouse gas emissions are substantiallyreduced, the long lifespan of CO2 in the atmosphere means that wecannot avoid further climate change due to CO2 already in the
atmosphere.Despite the uncertainties, all models show that the Earth will warm inthe next century, with a consistent geographical pattern.11
Having said that global warming is not just part of a natural cycle, there will always benatural variability in the climate – they are two different things. Human memory is shortterm e.g. snow in February 2009, doesn’t square with a lot of peoples concept of globalwarming and climate change. Similarly, people can always make reference to a yearwhen 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 (inthe way that they view it) in their lifetime, e.g. Climate change does not mean that therewill 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 theeffect of the media, and the human memory/experience. Global warming/Climatechange is a trend not a single weather event.12
Evidence shows rainfall patterns are changing across the globe. Generally, wet places arebecoming wetter and dry areas are becoming drier. However, there are also changesbetween seasons in different regions. For example, rainfall in the UK during summer isdecreasing, while in winter it is increasing. In the UK, the growing season has lengtheneddue to Spring starting earlier and the delayed onset of autumn/winter. Wildlife expertshave noted that many species are changing their behaviour, from butterflies appearingearlier in the year to birds starting to change their migration patterns.16
Long-term drought could cause huge migration and subsequent security issues17