1) GW is real but the origin is mostly not anthropogenic (the anthropogenic factor is greatly overestimated). The current average temperature (measured by the inclease of average world sea level) increase started around 1850 and is primarily due to changes in solar activity (the sun is getting older?)
2) The average earth temperature does not increase monotonically (reasons are still unknown).
3) Other causes (astronomical, solar, geophysical) can play an important role.
OTHER SOURCES: Climate Change Monitoring and Impacts Using Remote Sensing and Modeled Data,