Editor’s Note: Science is a world of theories and factions in which the truth ultimately emerges from vigorous debate. The events cited in this timeline are intended to paint a picture of Earth’s long history and how climate changes large and small have helped transform the planet. The dates and details here represent the general scientific and historical consensus, as supported by available data in 2005.

4.6 billion years ago

Earth forms as bits of space debris begin to accumulate, eventually growing large enough to develop a gravitational pull and an atmosphere.

600 million years ago

The world finally emerges from a series of tremendous glaciations known as the “snowball earth” period, in which even the equator was icebound. Some scientists espouse a controversial theory that the alternating cycles of global ice and volcanic activity somehow sparked the “Cambrian explosion the sudden evolution of thousands of new species between 570 million and 510 million years ago.

300 million to 280 million years ago

Earth experiences a major glaciation, characterized by very low concentrations of carbon dioxide in its atmosphere.

250 million years ago

A mass extinction, recorded in fossils, marks the beginning of the Mesozoic Era. Scientists believe that 90 percent of the species living at the time disappeared, to be replaced by new creatures – dinosaurs and small mammals.

65 million years ago

The end of the era of the dinosaurs. Today, many scientists support the theory that a massive asteroid struck Earth, filling the atmosphere with dust and debris for several years and causing a drastic but temporary climate change that few species could survive.

5.8 million to 5.2 million years ago

Man’s earliest-known ancestor, Ardipithecus ramidus kadabba, appears in Africa.

2 million to 1.6 million years ago

The beginning of the Quaternary Period, in which Earth’s climate continues to cool, introducing a long glacial cycle that lasts to the present. Glaciers have advanced into temperate zones at least nine times, each occurrence bringing at least 90,000 years of ice buildup, followed by rapid melting and around 10,000 years of relief known as an “interglacial” period. Geologic evidence of at least two of these glaciations exists in New England.

100,000 years ago

The first modern humans, Homo sapiens, appear in Africa.

75,000 years ago

Toba erupts in Sumatra, the largest volcanic event of the past half-million years. Its crater measures 60 miles across, and enough dust is thrown into the atmosphere to cool Earth for nearly 200 years. One model estimates that high noon on a day after the eruption would have been about as bright as a night with a full moon.

40,000 years ago

Neanderthals, an early hominid species, disappear from Europe, after coexisting with early humans and living for nearly 200,000 years. Some scientists have theorized that Neanderthals were unable to adapt to climate change as the ice age began and large game animals such as mammoths fled south.

35,000 to 30,000 years ago

Glaciers advance from the poles as the most recent ice age begins.

28,000 to 20,000 years ago

The Laurentide Ice Sheet, which blanketed New England during the most recent ice age, reaches its peak, stretching as far south as Long Island.

17,000 to 12,000 years ago

The last glaciers disappear from Maine.

10,000 years ago

The Holocene Epoch begins, a warm phase in geological history that permitted establishment of agriculture and the growth of human civilization. The Holocene, which continues today, is known as an “interglacial” period – an acknowledgment on the part of scientists that ice will return someday.

2200 B.C.

The Akkadian Empire in what today is Syria collapses in reaction to a drought that has been linked to climate change.

A.D. 750 to 900

The Maya civilization in present-day Mexico disappears. Scholars theorize many reasons for the collapse, including a significant drought throughout the hemisphere, which resulted from a shift in the El Nino-La Nina circulation.

985 to 1000

Leif Ericson lands in what is now Labrador. Viking explorers established colonies in Greenland and North America, as icebound ports melted during a brief climate shift known as the Medieval Warm Period. Most of these settlements were abandoned by the 1350s, when ice returned.

1315

The Little Ice Age begins, ushering in a period of at least 400 years in which average temperatures in Europe dropped significantly and wild weather wreaked havoc on the region’s agrarian societies. In 1315, cold and torrential rain caused a wheat shortage known as “The Great Famine,” which lasted for several years and killed hundreds of thousands of peasants.

1587

A small group of British colonists at Roanoke Island, off North Carolina, disappears. Climate records have revealed that the American coastline was experiencing an unusual drought – the driest growing season in 800 years.

1604

Samuel de Champlain and Pierre Dugua, Sieur de Mons attempt to found the first French colony in America in the middle of the St. Croix River. After a single year, however, and a harsh winter in which half the colonists died, the Frenchmen moved the colony to warmer shores, in present-day Nova Scotia.

1608

A British colony near the mouth of the Kennebec River fails after just one year when all 100 colonists either die or return to England. Popham Colony, originally founded as a sister settlement to more southerly Jamestown in Virginia, is beset by bad luck, including a particularly severe winter that came on far sooner than colonists had anticipated.

1629

The Indian monsoon, which regularly brings desperately needed rains to Asia, fails to occur for two straight years, causing a famine that kills thousands of people and millions of cattle in India. Some areas do not recover for more than 50 years.

1769

In Great Britain, inventor James Watt refines the steam engine, which becomes a major power source for the Industrial Revolution in Europe and America. Pollution from the coal used to fuel the new factories becomes a major factor in the increase of carbon dioxide in the atmosphere that begins during this period.

1777

George Washington and the Continental Army are snowbound at Valley Forge, Pa., a consequence of the Little Ice Age.

1815

Tambora erupts in Indonesia. The volcano directly kills 50,000 people, but cooling effects on the world’s climate linger for months, resulting in what has been called the “Year Without a Summer” in 1816.

1846

The Irish potato famine begins, with the unusually damp, cool weather pattern a major factor in the crop’s failure.

1878

The Staples family starts keeping records of ice-outs on West Grand Lake in Washington County. Descendants continue the practice today, providing one of Maine’s longest-running direct climate records.

1896

Swedish scientist Svante Arrhenius publishes a paper in which he coins the phrase “the greenhouse effect” and predicts that increasing the amount of carbon dioxide in the atmosphere would raise Earth’s temperature by several degrees.

1900

A Category 4 hurricane strikes Galveston, Texas, killing more than 6,000.

1913

American scientists use data from the 1883 eruption of Krakatoa in Southeast Asia to prove that volcanoes can affect global climate, a theory proposed by Benjamin Franklin more than a century earlier in response to his observations of climate impacts when Laki erupted in Iceland in 1783.

1932

Geologist W.J. Humphreys introduces the idea that Earth’s climate is not stable and that an ice age could return. “We are not wholly safe from such a world catastrophe,” he wrote.

1947

A team of scientists at the University of Chicago develops radiocarbon dating, a technique for determining the age of any organic item, such as a fossilized plant or a human bone, by measuring how its constituent carbon has changed with time. The technique can accurately date items as old as 50,000 years, and made paleobotany a key element of understanding past climate shifts.

1962

Bangor is snowbound as the year ends with a storm that delivers 20-foot drifts. The biggest snowstorm in recent memory, the Blizzard of ’62 cripples the city, reducing I-95 to a single lane and stranding more than 600 people in a local restaurant.

1970

The United States celebrates its first Earth Day, the symbolic start of the modern environmental movement, which helps to shift the discussion of climate change from academia to the public.

1980

Mount St. Helens erupts in Washington state, sending ash around the world over the course of a month. Neighboring Idaho and Montana briefly experience drastic temperature decreases because of ash and dust in the atmosphere.

1982

The world’s climate is affected by a particularly dramatic El Nino event. This ocean circulation pattern named for the Christ child tends to occur every four to 12 years, bringing drought to eastern Australia and Indonesia and heavy rains to western South and Central America.

1988

A heat wave and drought grip the Great Plains, resulting in a major fire at Yellowstone National Park. Climatologist James Hansen presents data to a U.S. Senate committee, proof that Earth’s climate had been warming for nearly a century, and testifies that he believes fossil fuels are the major cause.

1998

The warmest year of the 20th century occurs, based on global temperature averages.

The El Nino climate phenomenon hits a landmark during the winter of 1997-98, with various storms causing more than $10 billion in damage worldwide.

Maine is crippled by “the Ice Storm,” several days of sleet and hail that leave tens of thousands without heat and power. With several inches of ice encasing cars, doors and utility lines, it takes days for life in Maine to return to any semblance of normality.

2005

The Kyoto Protocol, an international agreement among industrialized nations to attempt to slow human-caused climate change, takes effect. The United States is not among its signatories.

Hurricane Katrina, a Category 4 hurricane, strikes the Gulf Coast of the United States, flooding the below-sea-level city of New Orleans. Bodies were still being found in December – four months after the storm struck – elevating the death toll to more than 1,300.