This is a very important issue. The work of Mann et al on the so-called hockey stick (a diagram which showed a fairly flat temperature for the Earth from about 1000 AD to the end of the 19th century and a strong increase in the 20th century) was publicised very strongly by the IPCC. It's a very convincing diagram with the only problem (for the IPCC) being that it is wrong. Two Canadian researchers, McIntyre & McKitrick pointed to flaws in it in a paper published in Geophysical Research Letters, a prestigious journal. The AGU, published of GRL, termed critical this paper as a Journal Highlght. Other scientists have supported the Canadians.  The basic point of the Canadians is that Mann's computer programs will always find a hockey stick, even if the data are random noise.
    When the Canadians asked Mann et al for for the computer programs and data he used for the paper, he refused to give it to them. This is against scientific ethics, particularly in view of the enormous amount of money being spent on "global warming". Because Mann et al's research funds came from the Federal Government and because Congressman Bartlett chairs an energy subcommittee,  the Congressman requested that Mann and some other researchers supply their data to people who asked for it. Details of this can be found on the web site climateaudit.org. I believe that Mann et al have supplied some of their data but not all.
    The Mann data look strange to me because they do not show the Medieval warm period (a time when the Vikings lived on Greenland for many years) and the Little Ice Age, a period when extreme cold in Englad, Europe and North America meant that crop yields were very poor. It is indisputable that these unusual events occurred.
   There are also problems with the warming of the earth's atmosphere due to CO2 emissions. The bulk of the warming is caused by water vapor and, at the present time, not all of the details of the spectrum of water vapor have been calculated.