Answer to Question 1
Scientists apply the principle of uniformitarianism. Assumptions about a temperature reconstruction using proxy data are only acceptable if the same proxy method works to estimate temperature in modern time where we have temperature measurements for comparison. The close matches of proxy temperature estimates and actual temperature measurements in Figure 21.9a and the right side of Figure 21.16 support the validity of proxy climate records to estimate ancient climate conditions.
Answer to Question 2
Although the total temperature change seems very small (0.7oC in 150 years), we can definitely say that it is real because the increase is larger than the uncertainty in the measurements and the process of averaging temperatures from many locations to determine a global average. It is seen to be significant when compared to proxy temperature reconstructions in Figure 21.16, which shows that a 0.7oC change in any 150-year period is highly unlikely at anytime in the last 1000 years.
Advantages and disadvantages of select temperature measurement sites
Average Global Temperature Rise over the Past Century
Homeostasis is analogous to regulation of temperature via a thermostat.
Thermodynamic Diagrams and Vertical Temperature Profiles
Climate change is an important vector for human diseases
Scientists joke