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Human actions—primarily burning fossil fuels, but also deforestation and other activities—have resulted in a rapid increase in the concentration of GHGs in the atmosphere. GHGs, including carbon dioxide, methane, and nitrous oxide, trap heat close to the surface and cause global temperatures to rise. Since the industrial revolution, CO2 concentrations have risen by more than 30%—currently 420 parts per million, up from 180 to 300 ppm historically over the prior several hundred million years. The result has been an average global temperature increase of approximately 0.2°C per decade. Today, mean surface temperatures are approximately 1.2°C (2.1°F) warmer than in the preindustrial period—enough to destabilize large ecosystems around the globe.
人类燃烧化石燃料以及砍伐森林等行为，导致大气中温室气体浓度快速升高。二氧化碳、甲烷和氧化亚氮等温室气体将热量俘获，并储存在地表，导致全球气温升高。自工业革命以来，二氧化碳浓度提高了超过30%，目前已经达到420 ppm（百万分率），而在历史上，二氧化碳浓度从180 ppm提升到300 ppm，用了数亿年时间。二氧化碳浓度提高导致全球气温平均每十年升高约0.2摄氏度。目前的平均地表温度较前工业化时期提高了约1.2摄氏度（2.1华氏度），足以在全世界造成大规模的生态系统动荡。
 
Scientists’ climate models have thus far been remarkably accurate at predicting global temperature rise. And forecasts have generally underestimated the speed and severity of the impacts of a changing climate—for example, glaciers and ice sheets—are melting far more quickly than most climate scientists expected even just five years ago. Today, many scientists see a range of once- unlikely climate tipping points—including the rapid collapse of ice sheets, thawing permafrost in the Arctic, and the disruption of critical ocean currents—as “too risky to bet against.” Any one of these (factors/tipping points/possibilities) is potentially cataclysmic in its impact, and is a plausible outcome within our lifetimes.
到目前为止，科学家们的气候模型都非常精确地预测了全球气温升高的结果，但预测结果普遍低估了气候变化的速度及其对生态影响的严重程度。例如，冰山和冰盖的融化速度甚至远远超过了大多数气候科学家五年前的预期。今天，许多科学家认为以前不太可能出现的各类气候临界点，例如冰盖快速崩解、北极永冻层解冻和关键洋流中断等都有出现的可能，因此我们“面临的风险太大，不可存有侥幸心理”。任何一种因素/临界点/可能性都会产生灾难性的影响，而且在我们有生之年似乎就能成为现实。