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Climates of the Past

Earth’s climate has been changing for billions of years. It warmed and cooled many times long before humans were around.

There weren’t any people on Earth millions and billions of years ago to describe what climate was like, but the Earth kept records of past climates in special ways. Sediments and fossils deposited millions of years ago provide a record of ancient environments. Thin layers of mud and sand that form at the bottom of lakes record seasonal changes. Bubbles of ancient air trapped inside glaciers record what the atmosphere was like. Tree rings show what climate was like for each year of a tree’s life.

If climate has always changed, then why are people concerned about global warming? People are concerned because the Earth is warming faster now than it has in the past as more greenhouse gases are released into the atmosphere. Warming is happening faster and life on Earth, including humans, may not have time to get used to the warming planet.

Happenings During the Paleozoic Era (545-248 Million Years Ago)



  • Early in the Paleozoic the continents were far apart, but moving tectonic plates caused continents to move together into one large continent (a supercontinent!) called Pangaea.


  • Glaciers formed 430 million years ago. They may have only lasted one or a few million years. During this time, ice covered the northern part of Africa, which was located over the South Pole.
  • Climate models have been used to help understand weather and regional climates of the supercontinent Pangaea. The models suggest that monsoons affected the subtropical east coast and that interior of Pangaea was dry.

Evolutionary Events:

  • According to fossils, a large number of animals (including the distant ancestors of many modern animals) evolved between 530 and 520 million years ago.
  • Invertebrates (animals without backbones) such as corals, brachiopods, mollusks, and arthropods such as trilobites ruled the oceans in the early and middle parts of the Paleozoic.
  • Vertebrates (including fish, amphibians, and reptiles) began to flourish in the later Paleozoic. Animals and plants populated the land (but many still lived in the ocean!).
  • About 440 million years ago the Late Ordovician Mass Extinction occurred. It was the second largest mass extinction of all time. Over 10 million years, many marine species became extinct including those that built reefs.
  • At the end of the Paleozoic, about 250 million years ago, as many as 96% of species in the oceans became extinct. They didn’t die all at once. It took over 8 million years for the mass extinction to wipe out all those species. This was the largest mass extinction of all time.
Characteristics of the Paleozoic
Click on image for full size
L.Gardiner/Windows to the Universe (fossil images courtesy of AGI Imagebank)

Happenings During the Mesozoic Era (248-65 Million Years Ago)

248 to 65 million years ago
(See the geologic timescale!)

At the start of the Mesozoic, the continents were all joined together forming one large continent called Pangaea. During Mesozoic time, they pulled apart from one another. Continents move due to plate tectonics.


  • The climate most likely remained warm throughout the Mesozoic. No evidence of glaciations has been found in Mesozoic age rocks and abundant evidence of tropical species has been found in Mesozoic age fossils.
  • During the last part of the Mesozoic (called the Cretaceous period) the climate warmed very much. Earth was several degrees warmer than it is today. There was much less variation in temperature between the equator and the poles at this time.
  • There is strong evidence that global cooling occurred at the end of the Mesozoic. The cooling may have been caused by either a huge asteroid impact near the Yucatan Peninsula, a large amount of volcanic eruptions in the area that is today India and Pakistan, or by a combination of both the asteroid and volcanoes. The Sun would have been blocked for some time by the debris spewed into the atmosphere.

Evolutionary Events:

  • Dinosaurs evolved and became abundant! Some were herbivores (eating plants) while others were carnivores (eating meat). Dinosaurs were reptiles, however there is some evidence that they may have been warm-blooded.
  • Birds: During the late Mesozoic, birds evolved from a group of small carnivorous dinosaurs.
  • Plants: Conifer trees evolved at the beginning of the Mesozoic. The first flowering plants evolved towards the end of the Mesozoic.
  • Mammals evolved during the Mesozoic but there were relatively few species and they were small in size. During the Mesozoic, mammals were eaten by carnivorous dinosaurs.
  • At the end of the Mesozoic, the Cretaceous-Tertiary Mass Extinction occurred. This was the extinction event that killed the dinosaurs (among others). Many of the animals and plants that survived the extinction event (such as mammals and birds) went on to become very abundant afterward. Likely causes of the extinction event include a large asteroid impact, erupting volcanoes, and climate change. There is evidence that all three of these happened.
Characteristics of the Mesozoic
Click on image for full size
L.Gardiner/Windows to the Universe

Happenings During the Cenozoic (65 Million Years Ago to Present)

Time:65 million years ago to today (and continuing!)
(See the geologic timescale!)


  • The Atlantic Ocean continues to widen as new ocean crust is formed at the Mid Atlantic Ridge.
  • India collided with the Asian continent forming the Himalayan Mountains which continue to grow higher today.
  • The African plate pushed into Europe forming the Alps.
  • In North America, the Rocky Mountains formed and the Colorado Plateau was uplifted.
  • Early Cenozoic climate was warm and humid and the climate cooled gradually during the Cenozoic.
  • During the past two million years, Earth’s climate has cooled and warmed over and over again. The cool times are called Ice Ages because large amounts of ice formed on land. The last time that these sheets of ice were very large was 20,000 years ago. Most of the sheets of ice melted by about 10,000 years ago.
  • Whenever climate cooled and ice sheets formed on land, there was less water left in the oceans, thus lower sea level. Sometimes lower sea level caused land that is usually underwater to connect continents. These areas of land, called land bridges, allowed animals to migrate to other continents.
Evolutionary Events:
  • Mammals, which had been small and few during the Mesozoic, became more diverse. New mammal species evolved and were able to live in areas and eat foods that had been used by dinosaurs during the Mesozoic.
  • Grass evolved and was well adapted to the cooler climates of the late Cenozoic.
  • Horses and other species of grazing animals evolved and ate the newly-evolved grass. The first horses were small, about the size of a labrador retriever.
  • Modern humans and their recent ancestors are called hominids. Thousands of fossils of hominids have been found; the oldest is more than 6 million years old. Fossils of hominids that are similar to modern humans are called Homo sapiens. Fossils of Homo sapiens are as much as 400,000 years old.

The Little Ice Age

The Little Ice Age was a time of cooler climate in most parts of the world. Although there is some disagreement about exactly when the Little Ice Age started, records suggest that temperatures began cooling around 1250 A.D. The coldest time was during the 16th and 17th Centuries. By 1850 the climate began to warm.

During the Little Ice Age, average global temperatures were 1-1.5 degree Celsius (2-3 degrees Fahrenheit) cooler than they are today. The cooler temperatures were caused by a combination of less solar activity and large volcanic eruptions. Cooling caused glaciers to advance and stunted tree growth. Livestock died, harvests failed, and humans suffered from famine and disease.

The Little Ice Age was not a true ice age because it did not get cold enough for long enough to cause ice sheets to grow larger. The cooling likely affected areas around the world but we have the most records of how it changed daily life from Europe. Some of the records and events that occurred during the Little Ice Age are listed below.

  • Fur trappers reported that southern Hudson Bay remained frozen for about 3 weeks longer each spring.
  • Fishermen reported large amounts of sea ice floating in the North Atlantic.
  • British people saw Eskimos paddling canoes off the coast of England.
  • Alpine (mountain) glaciers grew larger. In some cases, the ice engulfed mountain villages.
  • Winters were longer and growing seasons shorter according to tree ring data and records of cherry tree flowering.
  • Wet weather caused disease that affected people, animals and crops including the bubonic plague (also called the Black Death). This disease killed more than a third of Europeans.
  • Farms and villages in Northern Europe were deserted because the farmers couldn’t grow crops in the cooler climate. During the harshest winters, bread had to be made from the bark of trees because grains would no longer grow.
  • Limited crops and unhealthy livestock caused famine in areas of northern and Eastern Europe. Unlike today, there was no way to transport food around the world to areas where crops had failed and people were hungry.

Pieter Bruegel the Elder’s Hunters in the Snow (1565) depicts frozen rivers and lakes during the Little Ice Age. Scientists now say that decreased solar activity may increase the frequency of cold winters and return Northern Europe to such conditions.

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