To prove your point you will need to find a time when temperatures were higher then now, and supported a higher level of human life. Good luck with that! Otherwise your point that "A warmer wetter world might support more people," is utterly speculation.
The flaw in your argument is that this is going from a cold climate to a temperate climate, rather than from a temperate climate to a hot one. Humans favor a narrow climate range, and we are already on the high end of it.
"humans thrive best within a narrow "climatic envelope" around the world. Most of the world's population live in areas with a mean annual temperature of between 11 and 15 degrees Celsius (51.8 to 59 degrees Fahrenheit). A smaller band of between 20 to 25 °C (68 to 77°F) encompasses areas in South Asia that are affected by the Indian monsoon -- the annual rains that irrigates large swathes of cropland vital for food production.
Surprisingly, the scientists said, humans have favored living in these conditions for the past 6,000 years -- that's despite recent technological advances such as
air conditioning that have allowed us to push this boundary."
https://www.cnn.com/2020/05/05/world/global-warming-climate-niche-temperatures-intl-hnk/index.html
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Those who can will, those who can't will die presumably? Plus mass migrations historically speaking don't tend to be peaceful affairs. For example the end of Roman empire can be linked to climatic changes (in this case less stable and cooler conditions) which caused mass migration and crop failures. However if you look at the chart above, they weren't facing anything as rapidly changing as we are facing.
https://www.vox.com/the-big-idea/20...s-climate-change-disease-toppled-roman-empire
Regarding specifically climate change I leave you with the words of Sir David Attenborough (world renowned Naturalist & Broadcaster) "Right now, we are facing a man-made disaster of global scale. Our greatest threat in thousands of years. Climate Change."
Could it not be said Biden was caught out by the lie too which led to him supporting the Iraq War? Such as by a war mongering Republican president aided by dodgy intelligence from an ally (The British "sexed up reports"). Biden did later state he regretted supporting the war.
Okay lets discuss your indifference to all the lives being lost to climate change.
Which of these scenarios could save the most lives?
A. An opposition senator decides not to support the Iraq war (deaths from Iraq war estimates range from 151,000 violent deaths to 1,033,000). How many lives would have been saved?
B. The leader of the US provides clear and determined leadership when faced with a global pandemic (deaths from Covid 228,000 and still rising). He does not disband the pandemic task force, does not provide confusing health advise (?inject yourself with bleach), does not feud with state governors, does not downplay the danger of the virus despite knowing its dangers, does not feud with the scientists and health experts advising him (such as trying to discredit them), does not promote medications regardless of their actual benefits (see Hydroxychloroquine) and he does lead by example following recommended precautions for dealing with the pandemic such as social distancing and wearing a mask (rather then frequently doing the opposite). How many lives could have been saved?
C. The leader of the US takes a lead in combating climate change (deaths from Climate Change by 2050 estimated to rise to 6,500,000). He invests in sustainable energy, works to find ways to reduce carbon dioxide output (there are many). Maybe tariffs are placed on countries that continue to pollute heavily. He does not call climate change a hoax, he does not ignore reports proving it is happening, even those from his own administration, he does not pull out of the Paris accord, he does not promote fossil fuels, he does not pursue over 100 anti-environment policies and removal of regulations designed to protect the environment, he does not undermine the EPA, and he does not put a climate change deniers in charge of the EPA! How many lives could have been saved?
https://www.livescience.com/64535-climate-change-health-deaths.html
https://www.who.int/heli/risks/climate/climatechange/en/
By the way how do you feel about the fact that
Trump vetoed a senate bill that
Biden supported which wanted to end the US support for the humanitarian catastrophe that is the Saudi led war in Yemen? Doesn't really fit in with your narrative does it!
https://www.npr.org/2019/06/20/7344...-trump-senate-votes-to-block-saudi-arms-sales
My final question is more one of curiosity, are you against all wars, or just certain ones?