New ice core samples taken from the centre of the Greenland ice-sheet have given a detailed record of the last "interglacial (间冰期)" which rail from about 135,000 to 115,000 years ago. The cores, taken from a depth of 2,780 to 2,870 metres, show that during this period the climate oscillated(摆动) between three states instead of remaining in one, as in the whole of recorded human history. The middle state was like our own, but the others were either’’ much colder or warmer. Worse, it seems that the climate flipped from one condition to another very rapidly. "It apparently took very little time, perhaps less than a decade or two, to shift between the states," Dr. J.C. W. White of the Institute of Arctic and Alpine Research at the University of Colorado wrote earlier this year in the scientific journal, Nature: "We humans have built a remarkable socio-economic system during perhaps the only time when it could be built, when the climate was stable enough to let us develop the agricultural infrastructure(基础设施) required to maintain an advanced society." We do not know why we have been so blessed. But if the Earth had an operating manual, the chapter on climate might begin with a warning that the system has been adjusted at the factory for optimum comfort--so don’’t touch the dials. Unfortunately, we have been "twiddling the knobs (旋钮)" for decades. In December 1995 the official Intergovernmental Panel on Climatic Change ( IPCC）, which represents the work of 2,000 top meteorologists from around the world, concluded that global warming due to human activities is probably already taking place. Global warming sounds deceptively favorable to inhabitants of countries which currently experience harsh winters. In fact, with global warming, the world would struggle to cope with the effects of even a steady, gradual warming. This was spelt out to members of the British Royal Society by Sir John Houghton, chairman both of Britain’’ s Royal Commission of Environmental Pollution and of one of the main IPCC working groups. Houghton pat forward the IPCC picture of seas flooding much of Egypt, southern China and Bangladesh, "many millions" of people homeless; of horde’’s of "environmental refugees" and of wars breaking out over dwindling (becoming gradually smaller) fresh water’’ supplies, as world rainfall patterns changed. There is at least a chance that the world could adapt to steady warming if it happened slowly enough. However, many scientists, believe that even this prediction from the IPCC is too cautious. It can be inferred from the passage that global warming is______.