To expand on a point Sempie made, the colour of the ground determines how much light it reflects. White is the best reflector; black absorbs. This is why a black car will be hotter on the outside than a white car after a day in the sun.
This principle also applies to infra-red, which is the type of radiation via which we get our heat from the sun. White or near-white surfaces such as desert sand reflect a lot of heat. However, the reflected heat does not all go back out into space; some stays and diffuses evenly over the planet.
If you were to change the desert into a more fertile landscape, such as one of soil and trees, or fields for farming, your land would be hotter. However, the air near the land - I.e. the zone in which people walk around - would actually be cooler. This effect is why deserts are hot in the day and cold at night: during the day, the sand reflects the Sun's heat and heats the air, which is what people move around in and breathe. This makes it feel very hot. During the night, there is no heat to reflect and the ground has not stored any, so it gets very cold.
So, in summary, you wouldn't actually have a net temperature change. The difference between day and night, however, would be quite different, but this would make for very good farming land.
N.B.: Although the temperature wouldn't change as a direct result, other climatic effects will occur and may have an impact on temperature. See other answers.