If I would try to answer this question few years ago Id have to address many hypotheses and a surrounding discussion.However, the discussions on the main cause of the global warming we see nowadays have ended couple of years ago with results from new studies.

Indeed, our planet goes through natural cycles of global warming and global cooling (e.g. Ice ages). However, the speed of recent warming exceeds everything we have seen in previous global climate changes. Moreover, while we can explain previous climate changes with solar activity and seismological processes, these factors can not explain the rapid change we see in the last century. Basically, the only thing that can explain recent temperature changes on the planet is the increase of greenhouse gases concentration in the atmosphere. There are number of studies which proved that CO₂ gas is the one from many greenhouse gases, which concentration in the atmosphere changed significantly over the past century. So, up to this point there is pretty much an agreement within the scientific society: CO₂ increase in the atmosphere is the main factor of the recent global warming.

The next question is where is this additional CO₂ is coming from? Are humans responsible for this change? May this be a natural process? The unusual climate dynamic started at the same time as industrial revolution took place and it correlates well with CO₂ emissions human activity produced. So the discussion on the sources of CO₂ is also almost over. So, we are ~99% certain that human activity is the main factor which increases CO₂ in the atmosphere which causes an unusually rapid global warming.

At this point, the main focus should be on stopping greenhouse gas emissions from human activities. However, even if we would be able to cut all of them right now (which doesnt seem to happen), changes we caused are irreversible. Ice is melting, sea level is rising, and climate patterns are changing. There is a lot of uncertainty as to how exactly climate or sea level will look in few decades. The planet climatic system is extremely complex and we can not predict with sufficient certainty what will happen.

There are many ideas on how to reverse global warming but none of I am familiar with sound reasonable. E.g. there are ways to decrease temperature by increasing particles in the atmosphere. It is so called nuclear winter by bombing the Earth we can create an ice age. Due to complexity of the planet climatic system the consequences of such a solutions may be much more catastrophic than the global warming itself.

In summary, human activity over the past century caused an increase in CO₂ in the atmosphere, which is playing the main role in recent global warming. At this point we dont have a way to reverse these changes but we also dont know how the new warmed planet is going to look like.

The evidence suggests that the input of CO₂ gas to the atmosphere is the greatest anthropogenic (human-caused) contributor to the current trend in global warming. Currently, humans are globally contributing about 8 Gigatons (Giga means 10 to the ninth power) of carbon to the atmosphere per year. The greatest portion, estimated at 42% of it, is produced from burning oil. The best way to slow down the addition of CO₂ to the atmosphere is to decrease the amount of oil produced and burned, which isn't an easy task, but Santa Barbara is moving toward being "fossil free by '33".

The actual reversal of global warming is an even bigger obstacle. One method being considered is finding ways to remove CO₂ from the atmosphere and store it somewhere else: as a gas underground with an impermeable (CO₂ cannot move through it) cap rock on top, as liquid CO₂ in lakes on the ocean floor, in mineral form (for example as calcite, CaCO₃), or as plant material (the carbon in plant material originates from CO₂ in the atmosphere).