在机考形式下的
Lecture 2 (environmental science)
Narrator
Listen to a part of lecture in an environmental science class.
I’d like to take back about 11,000 years ago when earth enter the latest interglacial period. Interglacial periods are typically periods of time between ice ages when the climate warms and the glacial ice retreats for a time before things cool off again and another ice age begins. And for over the past several million years, earth sort of default climate has actually been ice age. But we have experienced periodic, regular falls. And the last one, the one we are in now started about 11,000 years ago.
Now the typical pattern for interglacial period and we’ve studied several is that the concentration of carbon dioxide and methane actually reaches its peak, that is there is the most carbon dioxide methane gas, greenhouse gases, in the atmosphere just after the beginning of the interglacial period. And then for reasons which not entirely clear, the concentration of greenhouse gases gradually goes down. Now the climate continues to warm for a while because there is a lag effect. But gradually as the concentration of greenhouse gases go down earth starts to cool again and eventually you slip back into ice age. However, for the latest interglacial period the one we’re in now, this pattern do not hold that is the concentration of the carbon dioxide and methane dipped a little bit after the peaking at the beginning, er near the beginning of the interglacial period but then it began to raise again. What was different about this interglacial period than the other ones? Well, one of the big differences is human activity. People began to raise crops and animals for food instead of hunting for them. This is the agriculture revolution and it began to happen in the earliest stages about 11,000 years ago. Now scientists have tended to regard the agriculture revolution as a beneficiary of the fortuitous shift in climate. However, some new theories of climate have proposed that perhaps humanity was having an effect on the climate as far back as the beginnings of the agriculture revolution. When you grow crops and pasture your animals, one of those things you do is you cut down the forests. If you cut down the forests, when you burn the trees for fuel and you don’t replace them with other trees or when you just leave them to rot and don’t allow other trees to grow you end up with lot more carbon and form of carbon dioxide getting into atmosphere.
Another gas associated with the spread of agriculture is methane. Methane forms in large concentrations above wetlands and as it turns out the cultivation of certain grains create that areas of artificial wetlands and properly drastically increase the amount of that thing getting into the atmosphere over and above what would be there. So, em… agriculture, the spread of agriculture, you know what we are talking over, thousands of years. But this could very well if had profound effect on the compensation of earth’s atmosphere. It’s kind of ironic to think that absent that effect, it maybe that we will be heading into an ice age again. In fact, back in 1970s’ a lot of theories were predicting that, you know, the climate was start to cool and we’d slowly enter into a new ice age. And then they were puzzled as to why it didn’t seem to be happening.
Now one of the implications for the future, well, it’s a little tricky. I mean you could say well. Here is an example, human activity, the agriculture revolution which actually with beneficial, we altered the climate for the better. Perhaps I am preventing an ice age. But then industrialization, of course, has drastically increased the amount of carbon dioxide that human is putting into the atmosphere, the burning of the fossil fuels tends to put a lot of CO2 into the atmosphere.
So we are entering into uncharted territory now, in terms of the amount of carbon dioxide, the concentrations of the carbon dioxide that now been put into the atmosphere as a result of industrialization and the use of fossil fuels.