Good news—we have extra time before the Sun ends life on Earth

好消息——在太阳终结地球生命之前，我们还有额外的时间

It’s a bit worrying when a scientific paper begins, “How long will life on Earth survive?” But in this case—a study by Jacob Haqq‐Misra of Blue Marble Space and Eric Wolf at the University of Colorado Boulder—the billion-plus-year timeline under consideration shouldn’t cause you too much existential panic. 当一篇科学论文以“地球上的生命还能存活多久？”作为开头时，确实会让人感到有些担忧。但在本案例中——由“蓝色大理石空间”（Blue Marble Space）的雅各布·哈克-米斯拉（Jacob Haqq‐Misra）和科罗拉多大学博尔德分校的埃里克·沃尔夫（Eric Wolf）共同进行的一项研究——所考虑的十亿年以上的跨度，不应让你产生过多的存在主义恐慌。

The context for this question is that we understand the Sun will brighten as it eventually matures into a red giant that swallows the Earth in a solar furnace. So, where along that 5 billion-year path will life on Earth, in fact, be cooked? 这个问题的背景是，我们已知太阳在演化为红巨星的过程中会逐渐变亮，最终将地球吞没在太阳的熔炉中。那么，在这50亿年的演化路径中，地球生命究竟会在哪个节点被“烤焦”呢？

Weathering and the weather

风化作用与天气

This isn’t just a question of incoming radiation. Among the thermostat-like stabilizing feedback loops in Earth’s climate, the cycling of CO2 through the solid Earth is a major factor over timescales this long. The weathering of silicate rocks at the surface converts atmospheric CO2 into carbonate that ends up on the seafloor, where it can be subducted into the mantle with tectonic plates. (And eventually, it can cycle back out to the atmosphere through volcanoes.) 这不仅仅是入射辐射的问题。在地球气候中类似恒温器的稳定反馈循环里，二氧化碳在固体地球中的循环是长期尺度下的一个主要因素。地表硅酸盐岩石的风化作用将大气中的二氧化碳转化为碳酸盐，最终沉积在海底，并随着板块构造俯冲进入地幔。（最终，它又可以通过火山活动循环回到大气中。）

The weathering of bedrock depends, in part, on temperature. Warmer temperatures and a more active hydrologic cycle mean an increased rate of weathering, which pulls more CO2 out of the atmosphere. That slows rising temperatures. But in this scenario, it could also lead CO2 to fall to extremely low levels—and photosynthesis requires CO2. 基岩的风化在一定程度上取决于温度。更高的温度和更活跃的水文循环意味着风化速率的增加，这会从大气中吸收更多的二氧化碳，从而减缓气温上升。但在这种情况下，它也可能导致二氧化碳水平降至极低——而光合作用是需要二氧化碳的。

This far-future puzzle has been the focus of many model simulations over the past few decades. With a steadily brightening Sun, when does the Earth either get too hot or too CO2-starved for the base of the food chain to survive? Some of those models have been relatively simple equations. Others have been more complex one-dimensional layer models, representing an ocean and an atmosphere separately in the math, for example. 这个遥远的未来难题是过去几十年中许多模型模拟的焦点。随着太阳持续变亮，地球究竟会在何时变得过热或因二氧化碳匮乏，导致食物链的基础无法生存？其中一些模型是相对简单的方程，另一些则是更复杂的一维层模型，例如在数学计算中将海洋和大气分开表示。

This new study brings a 3D model to the party and uses a pair of scenarios that mark opposite ends of a spectrum. The difference between the scenarios is based on extreme views of the temperature/carbon relationship described above. That was done in part because the idea that CO2 would eventually fall to very low levels has been challenged recently, based on some evidence of a much weaker relationship between bedrock weathering and global temperature. 这项新研究引入了一个三维模型，并使用了两个处于光谱两端的极端情景。这两个情景的差异基于对上述温度与碳关系的不同极端看法。之所以这样做，部分原因是近期有证据表明基岩风化与全球温度之间的关系远比预想的要弱，因此关于二氧化碳最终会降至极低水平的观点受到了挑战。

So in one scenario, the researchers held the planet’s temperature constant (equal to today’s) and let CO2 drop to compensate exactly for the brighter Sun. This is a world where strong weathering acts as a perfect thermostat. In the other scenario, CO2 is instead held constant at a modern value while temperature increases, representing a very weak weathering thermostat. 因此，在一种情景中，研究人员保持地球温度不变（等于今天），并让二氧化碳水平下降以精确抵消太阳变亮的影响。这是一个强风化作用充当完美恒温器的世界。在另一种情景中，二氧化碳保持在现代水平，而温度则随时间升高，这代表了一个非常微弱的风化恒温器。

Too hot to handle

难以承受的高温

With weak weathering, the world is around 21° C (38° F) warmer 1.5 billion years from now, and it jumps an additional 40° C (72° F) between then and 2 billion years. Even with CO2 remaining at 400 parts per million, those temperatures would wipe out land plants on Earth. Specifically, the physiological limits of most land plants are crossed by 1.68 billion years, and the rest are toast at 1.87 billion. (Boiling off the oceans and losing our water to space wouldn’t be far behind.) 在弱风化情景下，15亿年后地球温度将比现在高出约21°C，而在15亿年到20亿年之间，温度还会额外飙升40°C。即使二氧化碳保持在400ppm（百万分之浓度），这样的高温也会摧毁地球上的陆生植物。具体来说，大多数陆生植物的生理极限将在16.8亿年后被突破，其余的则在18.7亿年后彻底灭绝。（随后不久，海洋就会沸腾，水分也会散失到太空中。）

In the strong weathering scenario, the temperature doesn’t change. But after 1 billion years, CO2 drops to about 34 parts per million, and after 2 billion years it falls to less than 1 part per million. The limit for most land plants is around 150 parts per million, while the much less common C4 plants could survive down to 3–10 parts per million. The latter limit gets hit between 1.35 and 1.64 billion years in. A few plants, like cacti as well as some marine life, can cheat by using bicarbonate in the water if dissolved CO2 is low. They can probably make it down to 1 part per million. That would buy them a little more time, and they’d make it to about 1.84 billion years. 在强风化情景下，温度保持不变。但10亿年后，二氧化碳降至约34ppm，20亿年后降至不到1ppm。大多数陆生植物的生存极限约为150ppm，而较为罕见的C4植物可以在低至3-10ppm的环境下生存。这一极限将在13.5亿至16.4亿年之间被触及。少数植物（如仙人掌）以及一些海洋生物，可以通过利用水中的碳酸氢盐来“作弊”，从而在溶解二氧化碳极低的情况下生存。它们或许能坚持到浓度降至1ppm，这会为它们争取更多时间，使其存活至约18.4亿年。

That’s optimistic?

这算乐观吗？

The, uh, good news about these estimates for the demise of complex life on Earth is that they’re actually a bit more optimistic than most previous studies. That’s down to the 3D model producing a little less warming for a brighter Sun, the expectation that CO2 declines more slowly over time, and a slight expansion of the CO2 range believed to be survivable by plants. Many previous estimates had put life’s expiration date at less than 1 billion years from now. 关于地球复杂生命终结的这些估算，呃，好消息是它们实际上比以往大多数研究要乐观一些。这归功于三维模型在太阳变亮时产生的升温幅度较小，以及对二氧化碳随时间下降速度更慢的预期，再加上植物可生存的二氧化碳范围被认为有所扩大。许多之前的估算将生命的“保质期”定在距今不到10亿年。

Obviously, there are a bunch of additional considerations that could significantly alter this story, and the researchers mention a few. If civilization persists long enough to see some of these changes, geoengineering would certainly be an option—like spreading aerosols in the stratosphere to reflect sunlight, for example. There are even some wilder suggestions out there, like moving Earth’s orbit farther from the Sun or removing some of the Sun’s mass to tame the red giant. (We have a billion years to work on the logistics, after all.) 显然，还有许多其他因素可能会显著改变这一结论，研究人员也提到了其中几点。如果人类文明能持续足够长的时间来见证这些变化，地球工程无疑是一个选择——例如在平流层散布气溶胶以反射阳光。甚至还有一些更疯狂的建议，比如将地球轨道移至远离太阳的地方，或者移除太阳的部分质量来“驯服”这颗红巨星。（毕竟，我们还有十亿年的时间来研究这些后勤工作。）

Less speculatively, evolution could have a say in the physiological limits of Earth’s plants. Any adaptations that expand the range of survivability would extend the timeline. Ultimately, the point of modeling this kind of thing is not to make a confident prediction. Apart from the simple natural curiosity about what will happen to our world, this is also relevant to… 从非推测的角度来看，进化论可能会改变地球植物的生理极限。任何能够扩大生存范围的适应性演化都会延长这个时间表。归根结底，对这类事物进行建模的目的并不是为了做出确切的预测。除了对我们世界未来命运的自然好奇心之外，这也与……有关。