More than one experimenter has been fooled by his own data. The world's energy problems were solve by a team of researchers at the University of Utah when they discover cold fusion. That was until they found out that their data had fooled them. Measuring temperature can be tricky.
With a significant percentage of the surface of the Earth altered by mankind to produce food, shelter and ease of transportation, that should have an impact on climate. After all, CO2 is supposed to only make a 1 percent change, a 10 percent change or better in land use should have and impact greater than CO2. Some scientist agree than land use is responsible for most of the warming, some disagree. They are all good scientists, why would there be any disagreement?
Because numbers lie.
Anyone that has ever attempted to garden knows the value of mulch. It looks great when you first install it, it slows down the growth of weeds so it looks better longer, it retains soil moisture so you don't have to water as often, and it regulates the soil temperature. The mulch can be black, that nifty red looking stuff, honey colored hay, various shades of brown or even some goofy custom color to match your house color. No matter what color, it still does the job.
Forests tend to prefer the natural color mulch which is darker brown to nearly black. Most farmer don't use mulch. So if farmers remove trees and brush to build farmland, the natural mulch is turned into the soil. The temperature over the farmland will be warmer than the temperature over the forest floor and not just because of the shade from the trees.
Mulch, despite the chosen color is an insulator. The air trapped in the loose mulch warms before the soil and convects heat away from the dirt. Dirt is an insulator, but not as good as mulch. More heat is absorbed by the deeper soil uncovered by mulch than that covered. That is a good thing for seed germination,but it tends to increase evaporation of water from the soil. The more heat that is absorbed, the more watering that is required. Remember, that is another reason gardeners like mulch.
So soil temperature will be higher in the day time without mulch. The soil will release more of that heat at night because mulch is a better insulator than soil. This has a real and a not so real impact on the average global temperature.
The real part is that the soil absorbs more energy which is measured as increased temperature. The unreal part, is that the flow of energy and reflection of solar energy impacts the temperature measurements more than the air that is attempted to be measured.
Great pains are taken to make sure that the housings of the surface station temperature measurements are consistently white, so that they absorb a uniform amount of direct solar energy. Then that very scientifically designed instrument is mounted on a pole. In most areas, that pole is now galvanized metal. In some areas it may be pressure treated lumber. It others it may be a neat bracket. The choice makes a difference in the measured temperature.
Take a few of the new digital weather stations with the neat white beehive vented housing. Mount one on a black metal pole, one on a natural wood pole and one on a wooden pole painted white. Would there be any difference in the temperature measured? Now set each a white sheet under each, would there be a temperature difference?
So you see, hopefully, part of the issues with direct measurement of surface temperature.
What generally defines the energy absorption of the surface is the albedo or the reflectivity of the surface. What defines the impact of the albedo is the retained energy. A black surface that does not retain energy has little impact on climate by a great deal of impact on temperature measurement. This is the issue with determining how much impact agriculture has had on climate. The albedo change says not much. The difference in retained energy says a whole bunch. So a more accurate measurement of climate change would be soil temperature below the surface. That is not on the list of priorities, so the next best measurements are sea surface temperature and ocean heat content.
The moral of this story is take all measurements with a grain of salt. Everything measured needs verification if it is to be relied upon.