As the planet warms, it risks crossing catastrophic tipping points: thresholds where Earth systems, such as ice sheets and rain forests, change irreversibly over human lifetimes. Scientists have long warned that if global temperatures warmed more than 1.5 degrees Celsius compared with before the Industrial Revolution, and stayed high, they would increase the risk of passing multiple tipping points. With these warnings in mind, 194 countries a decade ago set 1.5 C as a goal they would try not to cross. Yet in 2024, the planet temporarily breached that threshold.
A tipping point is a metaphor for runaway change. Small changes can push a system out of balance. Once past a threshold, the changes reinforce themselves, amplifying until the system transforms into something new. The scientific reality of tipping points is more complicated than crossing a temperature line. Instead, different elements in the climate system have risks of tipping that increase with each fraction of a degree of warming. Climate scientist Timothy Lenton first identified climate tipping points in 2008. In 2022, he and his team revisited temperature collapse ranges, integrating over a decade of additional data and more sophisticated computer models.
For example, the beginning of a slow collapse of the Greenland ice sheet, which could raise global sea level by about 24 feet (7.4 meters), is one of the most likely tipping elements in a world more than 1.5 C warmer than preindustrial times. Some models place the critical threshold at 1.6 C (2.9 F). More recent simulations estimate runaway conditions at 2.7 C (4.9 F) of warming.
In the Amazon, self-perpetuating feedback loops threaten the stability of the Earth’s largest rain forest, an ecosystem that influences global climate. As temperatures rise, drought and wildfire activity increase, killing trees and releasing more carbon into the atmosphere, which in turn makes the forest hotter and drier still. By 2050, scientists warn, nearly half of the Amazon rain forest could face multiple stressors. That pressure may trigger a tipping point with mass tree die-offs. The once-damp rain forest canopy could shift to a dry savanna for at least several centuries.
Not all scientists agree that an AMOC or rain forest collapse is close. Despite the uncertainty, tipping points are too risky to ignore. Rising temperatures put people and economies around the world at greater risk of dangerous conditions. But there is still room for preventive actions – every fraction of a degree in warming that humans prevent reduces the risk of runaway climate conditions. Reducing greenhouse gas emissions slows warming and tipping point risks. Tipping points highlight the stakes, but they also underscore the climate choices humanity can still make to stop the damage.
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1.1. What is the key reality highlighted in the first paragraph?
A International cooperation is failing.
B A critical temperature limit was briefly exceeded.
C Scientists have abandoned the 1.5°C goal.
D Multiple tipping points were irreversibly crossed.
解析：选B。1. B 细节理解题。第一段强调的核心现象是：尽管设定了1.5°C的目标，但地球在2024年暂时突破（temporarily breached） 了这一阈值。B选项“一个关键的温度极限被短暂超越”准确概括了这一现象。A和C选项在文中未提及；D选项“多个临界点被不可逆转地越过”与段落内容不符，段落只提到风险增加，并未说已被越过。故选B。
2.2. According to the passage, a climate “tipping point” is best defined as a ________.
A precisely calculated temperature threshold
B point of no return for all Earth systems
C process of self-reinforcing system change
D sudden and unpredictable climate event
解析：选C。2. C 推理判断题。第二段将临界点定义为一种自我强化的、失控的变化（runaway change），一旦越过阈值，变化会自我加剧，直到系统转变为新状态。C选项“自我强化的系统变化过程”抓住了这个定义的本质。A选项“精确计算的温度阈值”与段落中“科学现实更为复杂”的表述相悖；B选项“所有地球系统的不可逆点”过于绝对；D选项“突然且不可预测的事件”并非定义核心。故选C。
3.3. The author mentions the Greenland ice sheet primarily to ________.
A highlight the uncertainties in tipping point forecasts
B illustrate its immediate and total collapse
C emphasize its isolation from global climate
D show the unanimous predictions of models
解析：选A。3. A 例证题。作者以格陵兰冰盖为例，给出了不同的模型预测（1.6°C 和 2.7°C），这主要是为了说明临界点预测中存在不确定性（uncertainties）。A选项正确。B选项“说明其立即且完全的崩溃”与“缓慢崩溃（slow collapse）”描述不符；C选项“强调其与全球气候的隔离”错误；D选项“显示模型的一致预测”与原文给出的不同数据直接矛盾。故选A。
4.4. What is the potential long-term consequence for the Amazon rain forest mentioned in the text?
A A permanent increase in its biodiversity.
B A shift to a much drier ecosystem.
C A rapid recovery after droughts.
D A stabilization of its carbon storage.
解析：选B。4. B 推理判断题。第四段指出，潮湿的雨林树冠可能转变为干燥的稀树草原（dry savanna），并持续至少几个世纪。因此，B选项“向一个更干燥的生态系统转变”是文中提到的潜在长期后果。A和D选项是积极变化，与文意相反；C选项“干旱后快速恢复”与“不可逆转变”的描述不符。故选B。
5.5. What is the author’s prevailing attitude towards addressing climate tipping points?
A Pessimistic resignation.
B Cautious optimism.
C Indifferent neutrality.
D Urgent advocacy.
解析：选D。5. D 观点态度题。在最后一段，作者强调临界点“风险太高不容忽视”，并明确指出“仍有预防行动的空间”，呼吁通过“减少温室气体排放”来做出“气候选择”。这些措辞体现了紧迫的倡导（Urgent advocacy） 态度。D选项正确。A选项“悲观的放弃”与作者主张行动的立场相反；B选项“谨慎的乐观”程度不足，作者的态度更偏向积极呼吁；C选项“漠不关心”完全错误。故选D。
