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 5 ______