What is the Ideal Temperature for Life on Earth?

There is a lot of talk about global warming these days. The thing I don't get is that over the earth's million years of being, it has both been much hotter and much colder than it currently is. So why do we think that a change in the Earth's temperature is necessarily bad?

So I propose we ask a slightly different question: what is the ideal temperature for life on earth?

If you think of it this way, it is not obvious at a quick glance whether global warming (or global cooling) would not in fact be better for life on earth.

In general I would think warmer would be better. If you look at the amount of life (or biomass) per square mile in the arctic and compare it with that of the tropics, there is much more life in the tropics. Why wouldn't we want the arctic region to warm up and allow more life to live there?

There would be tradeoffs if the temperature were to change. The polar bear would go extinct and we would lose 20,000 of them, but what if we could gain 30,000 black and grizzle bears as they are now able to live farther north? Would this be a good tradeoff? There would be more bears living, but one less species. How do you decide if this is good or bad?

To evaluate what the idea temperature would be, we should look at three things: biomass, biodiversity and biocomplexity. The goal would be to maximize all three understanding that there will be tradeoffs between them. How do you go about measuring these?

Biomass is the weight of everything that is alive in the world. Simple in concept, difficult in practice to measure. Seems like an important thing to get a handle on, and yet the various estimates on the internet are all over the place (part of the problem is the definition, where some just count living organisms, which is what I am talking about, and others look at living and dead forms, like oil and coal).

This Google Answer looks into it and has some good info.

Wikipedia puts it at 75 billion tons. Of this:
--humans comprise about 250 million tons (0.33%)
--krill, about 500 million tons (0.67%)
--farm animals, 700 million tons (almost 1%)
--crops, 2 billion tons (2.7%)

This site puts it at 1.8 trillion tons.

One goal should be to maximize the amount of life on the earth, and measuring the total biomass is the way to do it.

Biodiversity looks at how diverse life is. Biodiversity is valuable because it makes life more interesting. It is also valuable because it adds robustness to ecosystems. As situations change, the more diversity you have the better you are able to withstand it.

One measure of this would be the total number of species in the world. Once again, this seems like an important thing to know, and yet estimates of this are also all over the place.

World Watch: Estimated total number of species 4,000,000 to 100,000,000

Hypertextbook: 4 estimates from different sources: 2 - 50 million, 5-10 million, 30-50 million, 2-100 million

What they do say is that we have over 1.5 million named species. So we may know most of the species, or we may know less than 2% of all species. I wish they broke it down by animals, plants, insects, bacteria, etc. I bet we know most of the animals, especially the mammals.

Another aspect is the diversity inside a species. This is more difficult to measure, but maybe looking at the diversity of the genomes would help out here. While dogs are just one species, they show a great variety of diversity.

Some species show different cultures and traditions. Orca whales hunt different food with different techniques based on their pod and taught from generation to generation rather than being transferred by genes. Chimpanzees have different cultures. Some dolphins use sponges to protect their noses, a custom that is learned and passed down outside of genes.

Another goal should be to maximize the amount of biodiversity, in terms of species but also the diversity in species as well.

Biocomplexity is a measure of the complexity of an organism. 200 lbs of human is more valuable than 200 lbs of bacteria. One way to measure this is to look at the number of genes the species has. The more genes, the more complex and therefore more valuable. The number of cells in the body and the different type of cells also makes a creature more complex. Intelligence is also important. One way to measure this would be number of neurons in the brain, but this is not the most accurate as larger animals have larger brains but are not necessarily smarter.

The final goal would be to maximize biocomplexity. We should try and allow for the most amount of intelligent, complex organism and give them preference over the simpler forms of life.

With these three goalposts, scientists should figure out what temperature would allow for the most amount of life, taking into account diversity and complexity. We should then actively try and move the earth in this direction.