填空题

阅读下面这篇短文，短文后有2项测试任务：(1)第23～26题要求从所给的6个选项中为规定段每段选择1个正确的小标题；(2)第27～30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。
Human beings have little direct control over the volume of water in the atmosphere. But we produce other greenhouse gases that intensify the effect. The IPCC estimates that rising CO₂ emissions, mostly from burning fossil fuels, account for about 60 percent of the warming observed since 1850, Carbon dioxide concentration has been increasing by about 0.3 percent a year, and it is now about 30 percent higher than it was before the industrial revolution.
2. The relentless accumulation of greenhouse gases has led the IPCC to project that in the next hundred years average global temperatures will rise by 1 to 3.5 degrees C. That may not
seem like much. Yet the "little ice age," an anomalous cold snap that peaked from 1570 to 1730 and forced European farmers to abandon their fields, was caused by a change of only half a degree.
3. The compeer models used to project greenhouse effects far into the future are still being improved to accommodate a rapidly growing font of knowledge. And it is remarkably difficult to detect a definitive "signature" of human activity in the world’s widely fluctuating climate record. To project future climate patterns, scientists use computer simulations of the interactions among land, air, water, ice and sunlight. These general circulation models, or GCMs, consist of equations representing the known laws of atmospheric physics and ocean circulation. For each section of the planet, they calculate the effect of such factors as air temperature, the Earth’s rotation, surface friction at sea level, rainfall, and other climatic conditions. A perfect model, if given enough information about conditions on Earth several hundred years ago, could provide an exact description of today’s climate. Only very recently have models been developed that are capable of realistically depicting the present global climate without a lot of tinkering— adjustments often called "fudge factors."
4. In part, this is because only the most powerful computers are fast enough to handle the job, and in part because some aspects of climate change are still mysterious. Even avid proponents caution that GCMs are not yet trustworthy for predicting detailed effects in individual regions: Models divide the world’s surface into grids that are typically about 200 miles on a side, but ocean eddies, storms and cloud activity take place on far smaller scales. The modelers, therefore, have to compensate with approximations. According to Kevin Trenberth, chief climate analyst at the National Center for Atmospheric Research in Boulder, Colorado, all GCMs project global warming, but they can provide only a range of projected temperature change.
5. The role of clouds and airborne suspended particles called aerosols is no easier to factor into models. Clouds shade the Earth’s surface, promoting cooling. But, depending on their altitude, density, and other conditions, they can also trap outgoing heat, promoting warming. Aerosols are also equally tricky. Some encourage water vapor in the air to condense into tiny droplets. The resulting clouds are dense and shiny, shading the surface for weeks. Thus, ironically, our own pollution, mainly from combustion of sulfur-bearing coal and oil, may temporarily have spared us some effects of global warming.
6. Yet the warming could be part of the natural roller coaster of average global air temperatures, which have varied by as much as 6 degrees C during the past 150,000 years. Climate fluctuates over thousands of years owing to periodic changes in the sun’s energy output and in the Earth’s orbit and tilt, both of which influence the amount and intensity of sunlight reaching the surface. Proof of these climate shifts comes from variations in the composition of ice extracted in cores from the depths of ancient glaciers in Greenland and Antarctica and from differences among marine organisms in sediment cores taken from the seafloor. It is possible that around 1860, when scientists first began keeping dependable temperature records, the planet was still recovering from the "little ice age." The present warming might be a continuation of that rebound, and enhanced greenhouse warming may be superimposed on, and camouflaged by, that trend. Paragraph 6 ______