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Twenty thousand years ago, the Earth was a frigid landscape where woolly mammoths roamed.
两万年前，地球是猛犸象漫游的严寒地带。
Huge ice sheets, several thousand meters thick, encased parts of North America, Asia, and Europe.
几千米厚的巨大冰盖，覆盖了北美、亚洲和欧洲的部分地区。
We commonly know it as the "Ice Age." But geologists call it the Last Glacial Maximum.
这就是我们所知的“冰河世纪”。但是地质学家称之为“末次盛冰期”。
That's because it's the most recent time that ice reached such a huge extent,
原因是那是最近一次冰层达到如此巨大的时期，
and "ice age" is an informal term without a single agreed-upon definition.
并且“冰河世纪”是非正式用语，且没有单一的统一定义。
Over the last million years, there have actually been about 10 different glacial maxima.
在过去的一百万年里，实际上有大约十种不同的“盛冰期”。
Throughout Earth's history, climate has varied greatly.
纵观地球历史，气候变化很大。
For hundreds of millions of years, the planet had no polar ice caps.
数亿年来，地球并没有两极冰盖。
Without this ice, the sea level was 70 meters higher.
没有这些冰的时候，海平面比现在高70米。
At the other extreme, about 700 million years ago,
另一个极端现象大约发生在7亿年前，
Earth became almost entirely covered in ice during an event known as "Snowball Earth."
在这一被称作“雪球地球”的事件中，地球几乎完全被冰覆盖。
So what causes these massive swings in the planet's climate?
所以，造成地球气候变化如此之大的原因是什么？
One of the main drivers is atmospheric carbon dioxide, a greenhouse gas that traps heat.
一个主要因素是大气中的二氧化碳，它是吸收热量的温室气体。
Natural processes, such as volcanism, chemical weathering of rocks, and the burial of organic matter,
自然进程，例如火山活动、岩石化学风化，以及被掩埋的有机物质，
can cause huge changes in carbon dioxide when they continue for millions of years.
当这些进程持续上百万年，能使二氧化碳发生巨变。
Over the past million years, carbon dioxide has been relatively low,
在过去的一百万年里，二氧化碳相对较低，
and repeated glacial maxima have been caused by cycles in Earth's movement around the sun.
而这些重复出现的“盛冰期”是由于地球绕太阳公转造成的。
As Earth rotates, it wobbles on its axis and its tilt changes,
当地球旋转时，它沿着一条轴转动，并且变化倾斜角度，
altering the amount of sunlight that strikes different parts of its surface.
使不同表面接收不同强度的阳光。
These wobbles, combined with the planet's elliptical orbit,
这些转动与行星的椭圆轨道相结合，
cause summer temperatures to vary depending on whether the summer solstice happens when Earth is closer or farther from the sun.
导致夏天温度变化取决于夏至发生在地球离太阳更近或更远的地方。
Approximately every 100,000 years, these factors align to create dramatically colder conditions that last for millennia.
大约每十万年，这些因素产生持续数千年的寒冷环境。
Cool summers that aren't warm enough to melt the preceding winter's snow allow ice to accumulate year after year.
凉爽的夏天不够温暖，无法融化前一冬的雪，使得冰年复一年的累积起来。
These ice sheets produce additional cooling by reflecting more solar energy back into space.
这些冰盖通过将太阳热能反射回空中，使得周围更冷。
Simultaneously, cooler conditions transfer carbon dioxide from the atmosphere into the ocean,
同时，低温条件会将二氧化碳从大气转移到海洋中，
causing even more cooling and glacier expansion.
使得温度更低与冰川扩张。
About 20,000 years ago, these trends reversed
大约在2万年前，这些趋势发生逆转，
when changes in Earth's orbit increased summer sunshine over the giant ice sheets, and they began to melt.
当地球轨道的变化造成大冰盖上夏日阳光增加时，并且它们开始融化。
The sea level rose 130 meters and carbon dioxide was released from the ocean back into the atmosphere.
海平面上升了130米，二氧化碳从海洋中回到大气层。
By analyzing pollen and marine fossils, geologists can tell that
通过分析花粉和海洋生物化石，地质学家得出，
temperatures peaked about 6,000 years ago, before another shift in Earth's orbit caused renewed cooling.
在地球轨道的另一次转变导致重新冷却之前，温度大约在6000年前达到峰值。
So what's coming next? Based on the repeated natural cycle seen in the climate record,
接下来会发生什么呢？根据气候记录中的重复自然循环，
we'd normally expect the Earth to continue a trend of gradual cooling for the next few thousand years.
我们通常期望在接下来的几千年里，地球能保持逐渐降温的趋势。
However, this cooling abruptly reversed about 150 years ago. Why?
但此趋势在150年前突然逆转，为什么？
Carbon dioxide levels in the atmosphere have been rising since the 19th century, when fossil fuel use increased.
自19世纪以来，随着石燃料使用增加，二氧化碳含量开始升高。
We know that from studying air bubbles trapped in Antarctic ice.
我们是通过研究南极冰层里的气泡得知的这一点。
This surge in carbon dioxide also coincides with a global temperature increase of nearly one degree Celsius.
二氧化碳的这种激增也与上升近1摄氏度的全球气温相吻合。
Ice cores and atmospheric monitoring stations show us that carbon dioxide levels are rising faster,
冰核与大气监控站中显示，二氧化碳含量增长速度加快，
and to higher levels, than at any point in the last 800,000 years.
所达含量高于过去八十万年间的任何值。
Computer models forecast another one to four degrees Celsius of warming by 2100,
计算机模型预测到2100年将再升温1至4摄氏度，
depending on how much additional fossil fuel we burn.
这取决于我们的化石能源燃烧量。
What does that mean for the ice currently on Greenland and Antarctica?
这对格陵兰岛和南极目前的冰川来说意味着什么？
Past climate changes suggest that even a small warming shift can begin a process of ice melt that continues for thousands of years.
过去的气候变化表明，即使是小的变暖转变也可能开始持续数千年的融冰过程。
By the end of this century, ice melt is expected to raise the sea level by 30 to 100 centimeters,
到本世纪末，冰川融化预计使海平面上升30到100厘米，
enough to impact many coastal cities and island nations.
这足以影响许多沿海城市和岛屿国家。
If a four-degree Celsius warming persisted for several millennia, the sea level could rise by as much as 10 meters.
如果持续数千年4摄氏度变暖，海平面可升高10米。
By studying past climates, scientists learn more about what drives the shifts in ice that have shaped our planet for millions of years.
基于过去的气候研究，科学家们对导致我们星球塑造数百万年的冰的变化的原因有了更多了解。
Research suggests that by taking action now to reduce carbon dioxide emissions quickly,
研究指出，现在就采取行动，迅速减少二氧化碳排放量，
we still have the opportunity to curb ice loss and save our coastal communities.
我们仍然有机会遏制冰川减少，拯救我们的沿海地区。
