Not a single scientific study purports to know what the Global average temperature will be at any time in the future. The swiss cheese latice work of ambiguity, inneundo and non-sequitors that form the body of work underpinning the positions of the IPCC and similar groups do not even purport to be able to accurately predict future temperatures / impacts of increased CO2 levels, if they actually occur.
Al Gore is to Global Warming as a mutual fund salesman is to the stock market -- a lot of glib talk about TRENDS and STATISTICS appearing to support the sales pitch, but he/they havent a clue what the future will bring. The simple fact is that in a complex multivariate dynamic non-linear system, like climate or the stock market, you can spot all the trends and do all the regression analysis you want. The sixth tenths of one degree claimed increase in avergage global temperature over the last 100 years, and the claimed moderate increase in atmospherice CO2 over the last 350 years, for certain do not foretell a specific future relationship as cause and effect and correlation are not the same thing at all in a system such as Earths climate where at least 50% of the minor purported temperature increase over the last 100 years is due to non CO2 factors. Not even the IPCC has claimed to have scientific prove of the actual results of future CO2 increases if they occur. The have merely issued a set of projections wiht a 400% variance built in, and made such projection subject to numerous qualifications that they really have no idea what the effects of all of the interactions that may occur will be.
As rural areas have become urbanized over the last 350 years, certainly their historic temperature readings can not e fairly compared to what shows up today. This means that even the supposed six tenths of one degee GAT increase over the last 100 years is suspect.
If a rural area in the past 350 years has undergone urbanization, then it would be called "urban" in this study, don't you think?
And...
Show me a study that has ever even claimed that urban heat islands contribute to global surface average temperatures in a way that's not negligible.