There has been about 69 per cent decline in the wildlife population of mammals, birds, reptiles, amphibians, fish across the globe in the last 50 years. The highest decline, 94 per cent was in Latin America and Caribbean region. According to WWF report, Africa recorded 66 percent fall in wildlife population, the Asia Pacific 55 percent and population of freshwater species reduced by 83 percent globally.
The Sixth Mass Extinction: fact, fiction or speculation?
"Estimate that, since around AD 1500, possibly as many as 7.5–13% (150,000–260,000) of all ~2 million known species have already gone extinct, orders of magnitude greater than the 882 (0.04%) on the Red List."
We had a ~10C increase in temperature from 20000 YA - 13000 YA or so and then a yoyo jump back down ca. 6C and up again in a 1000 years, during the younger dryas.
While the 19th-21th century increase of 1-1.2C or so is a bit faster, it's not a magnitude faster.
Humans have definitely changed nature everywhere, and is probably responsible for many species dying. But blaming that on the climate change doesn't really make sense. Deforestation is much more likely to be the cause.
But deforestation and the following biodiversity loss is a completely different problem than reducing CO2. And in general, very few things that will reduce CO2 is going to help against deforestation (and contrary to popular opinion, even large scale reforestation will probably not affect CO2 very much either).
We are witnessing the sixth mass extinction event. Defined as the loss of 75% of species, this process typically spans around 2.8 million years. However, we're on track to reach this milestone in just about 100 years.
The abstract question of whether a higher or lower temperature would result in more habitable land (assuming, say, a gradual change over a million years) is completely 100% irrelevant to the issue of climate change.
A fast enough change will:
- Make places where large numbers of people currently live uninhabitable due to temperature and sea levels
- Kill off most plants and animals because the ones in a given place won't be adapted to the new climate, even if theoretically the climate in other places would now be suited to them, in a way that will take millions of years to recover from.
Simply saying "warmer is better" is utterly missing the point.
Well, if the earths population was 2 million, 15000 years ago and Sudan's today is 50 million, I think it's quite clear that Sudan's population has gone up quite a bit also over 15000 years.
edit: bryanlarsen pointed out that there was an even more extreme event in earth's history.