"An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains ice formed over a range of years. Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles (3.2 km), and contain ice up to 800,000 years old.
The physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core. The proportions of different oxygen and hydrogen isotopes provide information about ancient temperatures, and the air trapped in tiny bubbles can be analysed to determine the level of atmospheric gases such as carbon dioxide. Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past. These data can be combined to find the climate model that best fits all the available data.
“To say, ‘Is it the warmest for the last 100 years, or 1,000, or even 10,000 years?’ It’s a trickier question to answer,” Haustein said. “Before 1850 we didn’t have these observations, at least not enough to say something meaningful about the global mean.”
Researchers have documented temperatures from millions of years ago through natural sources like tree rings, ice cores, coral and lake sediments. This is the study of paleoclimatology.
Haustein said that comparing data collected through paleoclimatology to the temperatures this month suggests July could be the hottest month in 120,000 years.
How do you record the temperature from 120’000 years ago?
My guess would be Ice Cores.
From Wikipedia:
"An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains ice formed over a range of years. Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles (3.2 km), and contain ice up to 800,000 years old.
The physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core. The proportions of different oxygen and hydrogen isotopes provide information about ancient temperatures, and the air trapped in tiny bubbles can be analysed to determine the level of atmospheric gases such as carbon dioxide. Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past. These data can be combined to find the climate model that best fits all the available data.
This is from the article:
“To say, ‘Is it the warmest for the last 100 years, or 1,000, or even 10,000 years?’ It’s a trickier question to answer,” Haustein said. “Before 1850 we didn’t have these observations, at least not enough to say something meaningful about the global mean.”
Researchers have documented temperatures from millions of years ago through natural sources like tree rings, ice cores, coral and lake sediments. This is the study of paleoclimatology.
Haustein said that comparing data collected through paleoclimatology to the temperatures this month suggests July could be the hottest month in 120,000 years.