We have seen that there is clear evidence that greenhouse gas concentrations in the atmosphere have been rising since the a industrial revolution in the 18th century. The current scientific g consensus is that changes in greenhouse gas concentrations in the a atmosphere do cause global temperature change. However, the a biggest problem with the global warming hypothesis is i understanding how sensitive the global climate is to increased levels ? of atmospheric carbon dioxide. Even if we establish this, predicting climate change is complex because it encompasses many different factors, which respond differently when the atmosphere warms up, including regional temperature changes, melting glaciers and ice sheets, relative sea-level change, precipitation changes, storm intensity and tracks, El Ni~no, and even ocean circulation. This linkage between global warming and climate change is further complicated by the fact that each part of the global climate system has different response times. For example, the atmosphere can respond to external or internal changes within a day, but the deep ocean may take decades to respond, while vegetation can alter its structure within a few weeks (e.g. change the amount of leaves) but its composition (e.g. swapping plant types) can take up to a century to change. Then, add to this the possibility of natural forcing which may be cyclic; for example, there is good evidence that sunspot cycles can affect climate on both a decadal and a century timescale. There is also evidence that since the beginning of our present interglacial period, the last 10,000 years, there have been climatic coolings every 1,500 ±500 years, of which the Little Ice Age was the last. The Little Ice Age began in the 17th and ended in the 18th century and was characterized by a fall of 0.5-1°C in Greenland temperatures, significant shift in the currents around Iceland, and a sea-surface temperature fall of 4°C off the coast of West Africa, 2°C off the Bermuda Rise, and of course ice fairs on the River Thames in London, all of which were due to natural climate change. So we need to disentangle natural climate variability from global warming. We need to understand how the different parts of the climate system interact, remembering that they all have different response times. We need to understand what sort of climatic change will be caused, and whether it will be gradual or catastrophic. We also need to understand how different regions of the world will be og affected; for example, it is suggested that additional greenhouse | gases will warm up the poles more than the tropics. All these j| themes concerning an understanding of the climate system and the 3 difficulty of future climate prediction are returned to in Chapters 4 and 5.
So if you are reading this book for the first time and are primarily interested in the science of global warming then I would suggest you read Chapters 4, 5, 6, and 7- However, I would encourage you also to read Chapters 2, 3, 8, and 9, which look at the social, historic, economic, and political aspects of global warming, since global warming, as far as I am concerned, cannot be seen as a scientific problem; rather, it is a problem for our global society.
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