The correct answer is very much, “Because G-d said so in Bereshis.”
Depending on who one is speaking to in today’s world, that very well may not be enough so here is an alternative approach…
Rabbi Eliezer ben Hyrkanos discusses in his book Pirkei de Rabbi Eliezer Chapter 8, the intercalculation of the calendar. As he teaches us this intricate process handed down generation to generation he explains that the moon and sun pull on the earth – okay – science discusses this too, right?
However, he discusses how 7 planets pull on the sun for a period of time each and 7 planets pull on the moon for a period of time each. (Please refer to him for the intricate details.) Total of 84 years in a full cycle. Being that it is 5787 at the time of this writing and HaShem is basing His numbers on a lunar calendar, there have been 69 cycles and we are in the 75th year of the 70th cycle currently – again if HaShem is calculating this information based on a lunar calendar.
If global records were really being started to be kept around 1880 that means they have 145-146 years of data. Not to mention, where are they taking the temperatures from? Two cycles of 84 years is 168 years. Whether there is global warming or not, the fact of the matter is according to the tenets of science, there is not enough data to make a judgement.
From the NASA website, “The baseline used is the temperature average from the 30-year period 1951—1980. Every location is measured against that 20th century average.
Please remember that NASA calculates the lunar month to be 29.50358 days and Dovid Hamelech calculated it to be 29.53059, a difference of .000002 (The Kuzari Part II, Why We Need Astronomy translated by Avraham Yaakov Finkel page 96 footnotes). Dovid Hamelech used the same information Rabbi Eliezer was.