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个选项中为第2～5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。
Global Warming 1 Smoke is clouding our view of global
warming, protecting the planet from perhaps three-quarters of the greenhouse
(温室) effect. That might sound like good news, but experts say that as the cover
diminishes in coming decades, we are facing a dramatic increase of warming that
could be two or even three times as great as official best guesses. 2
This was the dramatic conclusion reached last week at a workshop in
Dahlem, Berlin, where top atmospheric scientists got together, including Nobel
prizewinner Paul Crutzen and Swedish scientist Bert
Bolin, former chairman of the UN’s
Intergovernmental Panel on Climate Change (IPCC). 3 IPCC scientists
have suspected for a decade that aerosols (浮质) of smoke and other particles from
burning rainforest, crop waste and fossil fuels are blocking sunlight and
counteracting the warming effect of carbon dioxide (二氧化物) emissions. Until now,
they reckoned that aerosols reduced greenhouse warming by perhaps a quarter,
cutting increases by 0.2℃. So the 0.6℃ of warming over the past century would
have been 0.8℃ without aerosols. 4 But the Berlin workshop concluded
that the real figure is even higher—aerosols may have reduced global warming by
as much as three-quarters, cutting increases by 1.8℃. If so, the good news is
that aerosols have prevented the world getting almost two degrees warmer than it
is now. But the bad news is that the climate system is much more sensitive to
greenhouse gases than previously guessed. 5 As those gases are expected
to continue accumulating in the atmosphere while aerosols stabilize or fall,
that means "dramatic consequences for estimates of future climate change", the
scientists agreed in a draft report from the workshop. The Berlin workshop concluded that the real figure______