The next issue is whether global warming is good or bad for a) the world as we know it and b) humans. Aside from the enthusiasts for having apple orchards in the Arctic and banana trees in Brooklyn (yes, I actually read that some years back--someone thinking that you could easily turn tundra into farms) the harm done to our existing ecosystem and human society and culture is clear.
Global warming has a negative effect on agriculture--our food source--in multiple ways. Successful agriculture depends on reliable frost-free dates, cooling hours (for crops that need cooling to produce fruit), length of growing season, and adequate fresh water. So variability in dates of first and last frost, number of cooling hours in winter, and rainfall intensity/frequency will have negative impacts on agriculture--including of course yields. Hotter weather increases water use by crop plants and livestock, requiring more water for success. At the same time, hotter weather increases evaporation from reservoirs that supply irrigation water and dries out the soil, reducing available soil moisture. At the extremes, heat reduces crop health--the plant cannot take in enough water to sustain growth. Extreme heat also affects how much fertilizer plants can utilize and how they react to pesticides. They are more vulnerable to plant diseases and pest attacks. As pest species move poleward, unhindered by winter freezes sufficient to knock back the population, crops are subject to more and more attacks. Poleward migration of species has already affected forestry (bark beetle infestations in southern Canada.) Desertification also causes the loss of former productive land, as does sealevel rise (which can contaminate soils with salt even before they go underwater.
Could we really use former permafrost in northern Canada and the newly uncovered barrens of Greenland to replace the agricultural land lost to desertification in the American Southwest, for instance? No, for multiple reasons. First, the warming climate is still unstable and thus relatively unpredictable. Second, as permafrost melts it doesn't create immediately useful farm soils...and much of Greenland is rock scraped bare or into rock flour by its ice cap. Some plants will grow there, but apple orchards and wheat fields are a long way away, if ever possible.
Rising sea level will have a negative impact on all coastal populations: that includes agriculture, urban development, and coastal industry. Coastal groundwater is already being invaded by salt water as the dual result of drawdown from its use and rising sea level, and coastal subsidence (from both human and other causes) is increasingly common. Pumping out of coastal aquifers and deep-pumping of petroleum have both been implicated. Much of the world's population lives within 100 miles of a coast...and low, sloping coastal plains (the Gulf Coast of the U.S., much of coastal India and parts of coastal China, for example) will suffer significant losses with even modest sea-level rise. Normal storm surges will cause increased coastal erosion, leading to more loss of land. The Gulf Coast, for instance, is thick with major petrochemical plants producing fuel and other products for the rest of the country. How hard would it be to move the infrastructure of the Gulf Coast inland 100 miles? Here's one on a water front.
Fresh water resources, even now barely sufficient for the human population, will be further limited by glacial melting: glaciers supply many of the highly populated river basins (including, in Europe, the Rhine.) They will also be affected by sealevel rise, as the saltwater invades coastal and near-coastal aquifers. And of course changes in rainfall distribution (not only in space but in time--and changing intensity/frequency) will affect freshwater availability.
Poleward migration of insect populations has a direct effect on human health, as does lack of hard freeze to kill off populations and allow them to expand. Tropical species, with the diseases they carry and the damage they can cause to human infrastructure, are a threat to human health.
Human populations thrive in temperate climates, not only because of the pervasiveness and severity of tropical diseases, but because like all mammals humans have a preferred temperature range in which their enzymes work optimally. A few degrees Celsius of fever--loss of homeostatis for temperature--and humans die. Even before that, their capacity to do useful work degrades. Heat injury kills.
So global warming has serious consequences for humans, as well as the existing ecosystems that have supported human populations throughout history. It will cut the amount of arable land, cut crop yields, cut the supply of fresh water, and increase pressure from pests and diseases.
Whether the warming is caused by human activity or not, it's bad for us, as the species we have become. The earth has been warmer than it is likely to become...but there were no humans on it at the time. We know humans can survive an Ice Age (they did) but we do not know if humans will survive a Hot Age. It would be worth trying to slow or reverse global warming even if we were sure it was not anthropogenic.
But we aren't.