If Earth’s gravity doubled? Would we survive, and what would life be like for ourselves and the rest of nature?
We might survive, but it would not be pretty.
An interesting (and wide-ranging) consequence would be – reduced tidal activity. Tides result from the pull of lunar gravity on the ocean. Increasing gravity would mean the planet would hold the oceans more firmly to Earth (though, I acknowledge that doubling gravity might also draw the moon closer, offsetting the effect). Less powerful tides would mean less erosion and a slowing of geological change. Continents and their features change far more slowly. As such, doubling Earth’s gravity would dramatically affect biological evolution by decreasing habitat variability. Although, only keen observers would be likely to notice such changes in the first ten years. In that time (provided this gravitational doubling did not simply tear the planet to pieces), we regular folk would see meaningful immediate effects.
For instance, flying animals would begin to behave much differently. The extra energy needed to support flight would demand extra strength (extra mass), and conversely, the extra mass would demand extra energy. You might start to see thousands of voracious birds walking around, trying to feed their flying habit. The costs would exceed the benefits, so most birds (i.e., those who did not simply starve) would stop flying, resort to strategic gliding, or come up with something else.
Flightless animals (that is us) would not have it much better. Smaller animals (and humans) would have an advantage since they need less energy to move efficiently in a strong gravitational environment. In successive generations (e.g., among species that produce 10+ generations of offspring in a typical 10-year span), the skeletal mass of animals would need to be proportionately higher to support whatever reduced height evolved. Similarly, muscle mass would need to be proportionately greater to support high mobility. Most animals would be slow-moving, with only high strength-to-weight ratio predator species keeping their relative speed. Species with low, flatforms and multiple supporting legs would also flourish – think centipedes.
So, one way to feel lighter is to float. Right? Well, it would not be all cake and ice cream for the swimmers either. Stronger gravity would favour bottom-dwelling species, as they are already well adapted to high-pressure living. For hundreds of aquatic species, however, the amount of energy needed to swim freely would exceed the amount of energy available from food floating freely (plankton), and free-floating food would sink faster. Here again, a flat form with larger skeletal mass would be the most effective. Saltwater offers more buoyancy than freshwater, so the changes would be more pronounced amongst freshwater species.
Finally, we would start to see changes in plant life as well. While height might be common, side branching would become difficult and expensive in terms of energy consumption. Plantlife would be unlikely to bend in the wind since they would need extra rigidity to support their weight. Once again, low growing, flat, well-supported forms would become dominant.
And of course, all these changes would happen within the delicate envelope of the planet, and 10 years is an instant in geological time. The extra gravitational pull of Earth would mean the atmosphere (like the oceans) would hug Earth much closer than it does currently. This added density would result in added heat, since molecules would collide much more often, resulting in more clouds and much more rain, along with severely reduced visibility.
So, there you have it. Ten years into your hypothetical future and there have been several massive extinction events, it rains all the time, and you have trouble seeing to the end of your driveway, but at least you never skipped leg day, so you’re still around to enjoy all the new moss and centipede salad joints that have opened. Please, do not feed the birds!