Temperature record of the last 2,000 years facts for kids
The temperature record of the last 2,000 years helps us understand how Earth's climate has changed. Scientists figure out past temperatures using clues from nature. These clues are called climate proxies. They combine these clues with modern temperature readings from thermometers.
Modern thermometers have only been used for about 170 years worldwide. So, to look further back, we need those natural clues. Studies covering the last 1,000 or 2,000 years show that recent temperatures are very unusual. The Intergovernmental Panel on Climate Change (IPCC) said in 2007 that temperatures in the Northern Hemisphere in the late 1900s were likely the warmest in at least 1,300 years.
A graph showing these temperature changes is often called the "hockey stick graph." This is because it looks like a hockey stick: a long, mostly flat part for many centuries, followed by a sharp upward curve in the last 100 years. Many different studies using various methods have supported this pattern. However, it's harder to know exact temperatures from very long ago because there are fewer clues.
Before the 1900s, the next warmest time was likely between 950 and 1100 AD. This period is known as the Medieval Warm Period. Some evidence also points to cooler times around the 1600s, called the Little Ice Age. Some people have questioned if the Medieval Warm Period was warmer than today. But scientific studies have shown that recent warming is much greater and more widespread.
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How Earth's Temperature Has Changed
According to the IPCC Sixth Assessment Report, human activities have caused global temperatures to rise. In the last 170 years, temperatures have reached their highest point in 2,000 years. The period from 2011 to 2020 was 1.09°C warmer than the late 1800s. Land areas warmed more (1.59°C) than oceans (0.88°C).
How Scientists Measure Past Temperatures
The best way to measure temperature is with thermometers. We have good thermometer records from about 1850 until today. These records show that global temperatures are generally getting warmer.
Before 1850, scientists use "proxy" records. These are not as exact as thermometers. They also don't cover as much of the Earth. But they are the only way to look far back in time. Since thermometers are more accurate, scientists use them when they are available.
Using Natural Clues (Proxy Data)
Since we don't have many thermometers from before 1850, scientists use natural clues. These clues are called proxy data.
- Tree rings: The width and other features of tree rings can tell us about past temperatures. Trees grow differently depending on the climate.
- Ice cores: Scientists drill into ice sheets and glaciers to get long cylinders of ice. These ice cores contain trapped air bubbles and different types of water molecules (isotopes). These can tell us about past temperatures and even the air's makeup.
- Lake sediments and corals: Layers in lakebeds and the growth patterns of corals also hold clues about past climate.
- Stalactites: These cave formations can also contain chemical clues about past temperatures.
Scientists compare these proxy records with thermometer readings from times when both exist. This helps them understand how the proxy relates to temperature. Then, they use the older parts of the proxy record to figure out temperatures from long ago.
To get a picture of global or hemisphere-wide temperatures, scientists combine many proxy records. They use special math methods to make sure one area with lots of tree rings doesn't unfairly affect the overall average.
In 2007, the IPCC looked at 14 different studies. Many of these studies covered 1,000 years or more. They all supported the idea that the late 1900s were the warmest in at least 1,300 years in the Northern Hemisphere.
Using Old Written Records
Scientists can also use old written records. These include things like:
- Records of when grapes were harvested.
- Notes about when harbors were free of sea ice.
- Diary entries describing very cold winters or hot summers.
These records can tell us if a certain region was warm or cold. However, they are harder to use for exact temperature numbers. They might also only be available for certain places or times.
Challenges in Measuring Past Temperatures
It's important to remember that these past temperature studies rely on certain assumptions. For example, they assume that the natural clues (proxies) change in a predictable way with temperature. Also, historical records might show local temperature changes, not necessarily global ones.
Scientists are always working to get more detailed information. They know that the further back in time they go, the less certain the temperature estimates become.
The Story of the "Hockey Stick" Graph
In the 1960s, a scientist named Hubert Lamb suggested there was a Medieval Warm Period and then a Little Ice Age. Early climate reports talked about this, but wondered if the warming was truly global.
Over time, scientists developed ways to use natural clues to estimate past temperatures. In the late 1990s, scientists Michael Mann, Raymond Bradley, and Malcolm Hughes created a new reconstruction. It showed global temperature patterns back to the year 1000.
This graph showed a long, fairly flat line for many centuries, then a sharp rise in the 20th century. A scientist named Jerry Mahlman called it the "hockey stick" because of its shape.
This "hockey stick" graph was shown in the 2001 IPCC report. It helped show that the 1990s were likely the warmest decade in 1,000 years in the Northern Hemisphere. This graph became very important in discussions about climate change.
Some people questioned the data and methods used to create the "hockey stick" graph. However, many independent studies have since confirmed the main idea. They used different data and methods, but they all showed a similar pattern: a relatively stable temperature for centuries, followed by a sharp increase in recent times.
The 2013 IPCC report also looked at temperature changes over the last two thousand years. It concluded that the period from 1983 to 2012 was very likely the warmest 30-year period in the last 800 years. It was also likely the warmest 30-year period in the last 1,400 years.
See also
- CLIWOC - A database of old ship weather records
- Dendroclimatology - The study of past climates using tree rings
- Table of historic and prehistoric climate indicators
