WHEN DID AGW HUMAN CAUSED CLIMATE CHANGE START?

1. Callendar 1938 [LINK] : It started in 1900 and warmed steadily from 1900 to 1938 with the warming driven by rising CO2 which in turn is attributable to fossil fuel emissions.
2. Hansen 1988 [LINK] : It started in 1950 because in the 30-year period 1950-1980 there is a strong measurable warming rate with 99% probability for human cause.
3. IPCC 2001: It started in 1750 when the Industrial Revolution kicked in and atmospheric CO2 began to rise.
4. IPCC 2015: It started in 1850 by when sufficient fossil fuel carbon had entered the atmosphere for a measurable response of temperature to CO2.
5. NASA 2020 [LINK] : It started in 1950 because from then the relationship between CO2 and temperature we see in the climate models closely matches the observational data.
6. Climate Scientist Peter Cox 2018 [LINK]  : It started in the 1970s because it is since then that we see a measurable responsiveness of surface temperature to atmospheric CO2 concentration according to the theory of the greenhouse effect of CO2.

WHAT SCIENTISTS KNEW FOR SURE IN THE 1970S

1. Harvard biologist George Wald:  Civilization will end within 15 or 30 years unless immediate action is taken against problems facing mankind.
2. Washington University biologist Barry Commoner: We are in an environmental crisis which threatens the survival of this nation, and of the world as a suitable place of human habitation,” [Published in the scholarly journal Environment.]
3. New York Times editorial:  “Man must stop pollution and conserve his resources, not merely to enhance existence but to save the race from intolerable deterioration and possible extinction.”
4. Paul Ehrlich, Stanford University: Population will inevitably and completely outstrip whatever small increases in food supplies we make. The death rate will increase until at least 100-200 million people per year will be starving to death during the next ten years. Most of the people who are going to die in the greatest cataclysm in the history of man have already been born and by 1975 food shortages will have escalated the present level of world hunger and starvation into famines of unbelievable proportions. Between 1980 and 1989, some 4 billion people, including 65 million Americans, will perish in the “Great Die-Off.”
5. Denis Hayes, Organizer of Earth Day wrote in the Spring 1970 issue of The Living Wilderness that “It is already too late to avoid mass starvation”.
6. Peter Gunter, Demography Professor, North Texas State University: “Demographers agree almost unanimously on the following grim timetable: by 1975 widespread famines will begin in India; these will spread by 1990 to include all of India, Pakistan, China and the Near East, Africa. By the year 2000, or conceivably sooner, South and Central America will exist under famine conditions….By the year 2000, thirty years from now, the entire world, with the exception of Western Europe, North America, and Australia, will be in famine.”
7.  LIFE MAGAZINE, January 1970:  Scientists have solid experimental and theoretical evidence that in a decade, urban dwellers will have to wear gas masks to survive air pollution…by 1985 air pollution will have reduced the amount of sunlight reaching earth by one half.
8. Kenneth Watt, Ecologist: At the present rate of nitrogen buildup, it is only a matter of time before light will be filtered out of the atmosphere and none of our land will be usable.
9. Barry Commoner:  Decaying organic pollutants will use up all of the oxygen in America’s rivers, causing freshwater fish to suffocate and disappear.
10. Paul Ehrlich, Stanford University: Air pollution will take hundreds of thousands of lives in the next few years alone and 200,000 Americans will die in 1973 due to “smog disasters” in New York and Los Angeles. DDT and other chlorinated hydrocarbons have substantially reduced the life expectancy of people born since 1945 and these people have a life expectancy of only 49 years. Life expectancy will reach 42 years by 1980, when it might level out.
11. Kenneth Watt, Ecologist : By the year 2000, if present trends continue, we will be using up crude oil at such a rate that there won’t be any crude oil left in the ground.
12. Harrison Brown, Scientist, National Academy of Sciences: Humanity will run out of copper shortly after 2000. Thereafter, lead, zinc, tin, gold, and silver reserves will all have been depleted by 1990.
13. Senator Gaylord Nelson & Dillon Ripley, secretary of the Smithsonian Institute: In 25 years (by 1995)  75% to 80% of all the species of animals on earth will become extinct.
14. Paul Ehrlich, 1975:  Since more than nine-tenths of the original tropical rain forests will be removed in most areas within the next 30 years or so, it is expected that half of the organisms in these areas will vanish with it.
15. Kenneth Watt 1970: The world has been chilling sharply for about twenty years. If present trends continue, the world will be 4C colder for the global mean temperature in 1990 and 11C colder in the year 2000. This is about twice what it would take to put us into an ice age.

WHAT SCIENTISTS KNEW FOR SURE IN THE 1980S

1. 1980, Since 1850 and the Industrial Revolution we have doubled atmospheric CO2 and if we continue to burn fossil fuels it could double again in the next fifty years (2030) because fossil fuels produce carbon dioxide faster than plants can absorb them. Warming could cause the West Antarctic Ice Sheet to crack and slide into the ocean to raise sea levels by 16 feet and submerge Florida.
2. 1981: Institute for Space Studies, NYC: Rising carbon dioxide levels in the atmosphere could bring a global warming of unprecedented magnitude melting the polar ice caps and flooding lowlands in the next century. The temperature rise could be 2.5C to 4,5C depending on the growth in fossil fuel consumption. A doubling of CO2 will cause a temperature rise of 3.3C. The West Antarctic Ice Sheet is vulnerable to rapid disintegration and melting. A global mean temperature rise of 2C could cause a rise of 5C at Antarctica melting the Ice Sheet and raising sea levels by 15 to 20 feet and flooding 25% of Florida and Louisiana.
3. 1982: The use of fossil fuels will cause atmospheric carbon dioxide to double in the next 40 to 100 years raising temperatures by an average of 2.8C by virtue of the greenhouse effect because carbon dioxide traps heat. The warming will cause polar ice to melt. In high northern latitudes spring will come earlier and earlier and winter later and later causing a decline in soil moisture. Warmer temperatures and less rainfall will devastate agriculture in much of the United States and the Soviet Union but a more regular monsoon pattern in India will increase rice production. Glaciers will melt and raise sea levels.
4. 1982, NOAA:  Two NOAA scientists published a paper in Science to say that in the period 1940-1980 50,000 cubic km of polar ice has melted by global warming and the sea level has risen by thermal expansion as well as the added water from the ice melt.
5. 1983, EPA : The world is powerless to prevent a greenhouse effect that will dramatically alter food production and living patterns. Instead of fighting the inevitable world leaders should be planning how to cope with its catastrophic impact. Coastal cities without sea-walls will be flooded. The climate of NYC will be like the climate now found in Florida. The US wheat belt will move northward. All because of global warming caused by a buildup of carbon dioxide emissions from fossil fuels. By the year 2100 these changes will produce catastrophic results. We should respond to this challenge with a sense of urgency. The warming process now set in place is irreversible and the dire predictions of global warming can only be delayed by a few years even with Draconian restrictions on fossil fuels. By the year 2000 the temperature could be 1.1 degrees higher, 3.6 degrees higher by 2040, and 9 degrees higher by 2100. The temperature rise in the poles will be three times higher melting the polar ice caps and causing sea levels to rise 3.5 inches by 2000, one foot by 2025, and five feet by 2100.
6. 1984:  Global warming caused by carbon dioxide pollution will cause noticeable warming by 2000 and increase the evaporation rate of water causing the level of the Great Lakes to drop 30% by 2050. These changes will cause a prolonged severe drought that will turn the American prairies into a dust bowl in the next few decades.
7. 1985, Roger Barry, Univ of Colorado data center for glaciology. Atmospheric CO2 will double by the end of the century due to burning fossil fuels. CO2 induced warming will be evident in the 1990s particularly in the melting of glaciers and polar ice caps. Glacial melting in the last century is explained primarily by global warming. There is a possibility of a seasonally open Arctic (after the summer melt) in the next century brought about by a doubling of atmospheric CO2.
8. 1985, Carl Sagan testimony in Senate hearing. Global warming will flood coastal cities and turn Midwest farmlands into a dust bowl. The greenhouse effect makes life possible but too much or too little will kill it off.Use of fossil fuels is pushing earth into too much. The answer is to reduce fossil fuel consumption by switching to nuclear and solar. If we do nothing we condemn our children and grandchildren to the effects of global warming. The greenhouse effect of fossil fuels is the most dangerous threat to mankind we have ever faced.
9. 1985: The Polar Research Board of the National Academy of Sciences: The sea level will rise 4-6 cm by 2000 and 12-27 cm by 2030 because global warming from the greenhouse effect will warm the oceans and melt glaciers and polar ice caps including Greenland.
10. 1986: Scientists at Senate subcommittee hearing: The greenhouse effect will cause the earth to be warmer in he next decade than at any other time in the last 100,000 years and cause shoreline erosion, droughts, and other catastrophic changes just as the depletion of the ozone layer is doing.
11. 1988, James Hansen of NASA tells the US Senate Energy and Natural Resources Committee that “the earth has been warmer in the first five months of this year than in any comparable period since measurements began 130 years ago” and therefore that the effects of greenhouse gas emissions from fossil fuels are now palpable. The nightmare has arrived. Humans burning fossil fuels have altered the global climate in a manner that will affect life on earth for centuries to come”. Southeast and Midwest states in the USA will experience frequent episodes of very high temperatures and drought in the next decade and beyond.
12. 1988, A buildup of carbon dioxide from the burning of fossil fuels emitted by human activities into is causing the earth’s surface to warm by trapping infrared radiation from the sun and turning the entire earth into a kind of greenhouse – just as mathematical models had predicted. Sometime between 2025 to 2050 the earth will be as much as 5C warmer with higher latitudes 11C warmer. Melting glaciers and polar ice and thermal expansion of the oceans will cause the sea level to rise of 4ft by 2050.
13. 1988, The hottest years on record occurred in the 1980s with the first 5 months of this year very hot. Just as the models had predicted, the rise in temperature is greater in high latitudes than in low, is greater over continents than oceans, and there is cooling in the upper atmosphere as the lower atmosphere warms up. Clearly, global warming by greenhouse gas emissions as predicted by these computer models has begun. “We can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and observed warming.” The snow is melting earlier each year and the rain belt is moving northwards.
14. 1988: To mitigate global warming we must reduce the use of fossil fuels that produce carbon dioxide and also agree to further reductions in CFC beyond the 50% reduction mandated in the Montreal Protocol. The global warming problem is real because ”We know that greenhouse gases are accumulating and in principle, they should lead to a global warming”.
15. 1989, We need a sharp reduction in the use of fossil fuels that produce carbon dioxide, and end to deforestation, and a program of reforestation. The re-development of nuclear power could also slow global warming. The world must immediately ratify a treaty to reduce the use of chlorofluorocarbons because they destroy ozone and contribute to global warming.
16. 1989, Scientists are using powerful computers and advanced mathematical models to simulate the world’s climate. The computer models predict that the greenhouse effect will make the earth warmer. The resulting climate change will have “important consequences for life on earth”. One problem is that the models don’t agree on what areas will suffer drought and where there will be increased precipitation. The dilemma faced by policymakers is that they don’t have information that is precise enough to make policy but if they wait for more precise information it may be too late to take effective action.

WHAT SCIENTISTS KNEW FOR SURE IN THE 1990S

1. 1990, NYT: Global warming will cause serious environmental damage starting early in the next century long before the maximum predicted temperature is reached. We must set limits beyond which the global temperature and sea level should not be permitted to rise to avoid serious and ever increasing risks posed by the continued flow of heat trapping gases into the atmosphere at present rates. The IPCC report serves as a prelude to the Second World Climate Conference in Geneva later this year.
2. 1990 A UN panel of international climate experts came out to strongly support the global warming theory saying that the buildup of CO2 from fossil fuel consumption lead to rising temperatures worldwide, altered weather patterns, lower food production, and rising sea levels. In the long run the cost of inaction exceeds the cost of mitigation. The panel put political pressure on President Bush who is not inclined to take costly measures against CO2 as long as there are credible scientists who oppose the global warming theory and as long as there is no “scientific consensus” on the issue. 
3. 1991, The National Academy of Sciences says US should act quickly to reduce carbon dioxide emissions by developing new generation nuclear power plants and by implementing reforestation, mass transit, and higher fuel efficiency standards for cars. The plan represents a compromise between the more extreme positions of the EPA and the Bush administration. Despite great uncertainties, global warming poses a threat sufficient to merit a prompt response. 
4. 1991, Burning fossil fuels produces aerosols that reflect sunlight and cool the earth. The resulting rise in temperature could more than offset the cooling achieved by reduction in CO2 emissions in the next 10 to 30 years according to an article in Nature by Prof Wigley, a climatologist at the University of East Anglia in England. The aerosol effect is a sleeping giant because it is something that has been missed and its effect is not trivial. It implies that reducing fossil fuel consumption will cause acceleration in global warming for 10 to 30 years before the gains from CO2 emission reduction kick in. 
5. 1991, NYT, The National Academy of Sciences says that the cost of inaction is not high because the US can easily adapt to the effects of global warming due to the greenhouse effect of pollutants in the atmosphere. It is more costly to control the climate change than to adapt to it. Human adaptability has been grossly underestimated. A dissenting committee member said that indirect costs of global warming have not been adequately considered. The report said it might be harder for developing countries to adapt to global warming. It encouraged “efforts to advance regional mobility of people, capital and goods,” better preparations for disaster and famine relief and expansion of free-market economies, so that changing prices can serve as market signals that would encourage people to adapt to global warming. 
6. 1992, TREATY TO CURB GLOBAL WARMING. Sixty nations sign an agreement at the Earth Summit in which they promise to reduce greenhouse gas emissions to 1990 levels. The agreement is not binding and there is no time table. 
7. 1992, WHITE HOUSE VOWS ACTION TO CUT GLOBAL WARMING GASES. The concentration of greenhouse gases is growing because of human activity and that could lead to catastrophic warming of the earth in the next century. Global warming advocates say that this move is positive and shows that the US has abandoned the flat earth society of global warming deniers. 
8. 1993, THE NEW YORK TIMES DEFENDS GLOBAL WARMING. There are two undisputed facts about global warming – carbon dioxide produced by burning fossil fuels has been accumulating in the atmosphere for a hundred years, and carbon dioxide traps heat reflected from the earth’s surface that would otherwise radiate out to space. It only remains to compute exactly how much the earth will heat up after an amount of CO2 is injected into the atmosphere. Since a real world experiment is not possible it must be carried out in mathematical models on supercomputers that simulate the earth’s climate although these models are far from perfect. Scientists have examined the results from the best computer models and advised the UN that CO2 will double by 2100 and cause a temperature rise of somewhere between 3C and 5C. 
9. 1993, RISING SEAS A PRECISE MEASURE OF GLOBAL WARMING. Using the most accurate system ever devised for measuring global sea levels,scientists have found a steady rise of 3 mm per year for the past two years. These data now establish beyond any doubt that the greenhouse effect is causing global warming. If this trend continues for another few years it will be solid evidence of a warming trend related to increases in atmospheric carbon dioxide. Doubts about the reliability of older and less precise temperature data may now be put aside as the very accurate sea level data clearly establishes the scientific basis of global warming. 
10. 1994, GLOBAL WARMING MAY HELP U.S. AGRICULTURE,(NYT). Civilization is playing a high stakes game with mother nature by emitting heat trapping greenhouse gases that could forever alter our fragile ecosystem in catastrophic ways. The planet is going to get hotter with radically altered weather and rainfall patterns. Yet, a new study appears to show that its effect on American agriculture will not be the dust bowl catastrophe that was once predicted. In fact, global warming is now expected to benefit American agriculture by greatly increasing crop yields. 
11. 1995, GLOBAL WARMING RESUMED IN 1994, CLIMATE DATA SHOW
    After a three year hiatus and a bitter winter in 1993-1994, the warming trend has returned with a warmer than usual winter in 1994-1995. Global warming is not gone, it was just temporarily interrupted by the 1991 eruption of Mount Pinatubo.
12. 1995, NEW EVIDENCE POINTS TO HUMAN ROLE IN GLOBAL WARMING
    Global warming will bring altered crop growing seasons, more severe storms, more tropical diseases, and the inundation of low lying areas by rising seas. As to the cause, the scientific debate about whether the warming is a natural variation or caused by man has now been settled. A scientific consensus due to advances in computer modeling has emerged that the cause of the warming is the greenhouse effect of carbon dioxide emitted by man’s fossil fuel consumption. This finding is issued in a new report of the UN-IPCC panel of scientists and is based on the best data and science available. These findings are now beyond question. The debate as to the cause of the warming in the last 100 years is now over.
13. 1995, GLOBAL WARMING JURY DELIVERS GUILTY VERDICT (New Scientist)
    UN-IPCC scientists issued a report in Dec 1995 saying that the warming of the earth by 0.5C in the last 100 years is the biggest since the last ice age and is not within the range of natural variability. Therefore it must represent a man made influence on global climate.Periods of cooling during the overall warming period can be explained in terms of global warming.
14. 1996, GLOBAL WARMING TO BLAME FOR BLIZZARD
    Just four days after scientists announced on Jan 3 1996 that global temperatures had crept to a record high in 1995, the Northeast US was hit by record cold and snowfall but scientists say that the blizzard of 1996 as well as the bitter cold in Europe were actually caused by global warming because warming increases evaporation that in turn increases precipitation.
15. 1996, GLOBAL WARMING POSES THREAT TO PUBLIC HEALTH
    (NYT) According to UN scientists, there are serious threats to public health if actions to reduce global warming come too slowly. The earth’s climate will change rapidly in the coming century as greenhouse gases trap solar radiation. Thousands could die in major cities in heat waves and tens of millions will face malaria epidemics in areas where the disease does not now occur. Last July a heat wave killed 465 people in Chicago alone.
16. 1997, THE BBC MAKES THE CASE FOR THE KYOTO PROTOCOL
    Twenty years of hard data from meteorological stations and nature show a clear warming trend. Growth rings in Mongolian and Canadian trees are getting wider. Butterflies in California are moving to higher ground once too cold for butterflies. Stalactites in Britain are growing faster. The growing season for crops in Australia is getting longer. Permafrost in Siberia and Canada is melting. The evidence is there anywhere you look.
17. 1997, WORLD VIEWS ON GLOBAL WARMING (LA TIMES)
    Entire nations among the Pacific islands vanish beneath the waves, coastal communities in the USA from North Carolina to the Texas Gulf wash out to sea, wild swings in precipitation first bring drought and then torrential rains and floods, coastal mudslides in California become routine, and maple trees of the North die out as dengue fever and mosquito borne encephalitis move in. 
18. 1997, SCIENTISTS WARN KYOTO DELEGATES
    “Without reductions in greenhouse gas emissions, scientists warn that carbon dioxide in the atmosphere could double in the next century, warming the atmosphere and triggering an environmental chain reaction that could raise sea levels, change ocean currents and intensify damage from storms, droughts and the spread of tropical diseases” (CNN).
19. 1998, WEATHER TREND IS PROOF OF GLOBAL WARMING
    Last year was the hottest year on record and this decade has already produced 9 of the 11 hottest years of the century. The data show that man made greenhouse gases are causing a potentially disastrous warming of the earth.
20. 1998, WORLD DEBATES GLOBAL WARMING
    Climate scientists in the Hadley Center on Climate Change: 1998 will be UK’s hottest year since 1106, “the warmest year of the millennium”; sometime between 2041 and 2070 we will see a sharp rise in sick, hungry, and thirsty people; by 2048 the world’s forests will become so degraded that they will change from net CO2 sinks to net CO2 producers further accelerating global warming; human greenhouse gas emissions have contributed substantially to global warming over the past half century; the climate model is validated by its ability to reconstruct the last 150 years of climate conditions; the 1997-1998 ElNino is the most extreme on record; in the next 100 years global temperatures will rise by 6C – the most extreme in the last 10,000 years. The Amazon forest will die out and rot releasing carbon dioxide. Tropical grasslands will be transformed into deserts. For the first half of the 21st century, vegetation will absorb CO2 at a rate of about 2-3 GtC per year while human emissions of CO2 are about 7GtC a year. From 2050 onwards, vegetation dying under the impact of climate change will itself add about 2GtC a year to greenhouse emissions, further intensifying global warming. Global warming will accelerate due to positive feedback. More than 170 million people will suffer from water shortage. The overwhelming consensus of scientific opinion is that climate change is real.
21. 1999, STUDY SHOWS ARCTIC ICE SHRINKING BECAUSE OF GLOBAL WARMING. Sea ice in the Arctic Basin is shrinking by 14000 square miles per year “probably” because of global warming caused by human activity according to a new international study that used 46 years of data and sophisticated computer simulation models to tackle the specific question of whether the loss of Arctic ice is a natural variation or caused by global warming. The computer model says that the probability that these changes were caused by natural variation is 1% but when global warming was added to the model the ice melt was a perfect fit. Therefore the ice melt is caused by human activities that emit greenhouse gases.
22. 1999, SURPRISE THEORY BEHIND BIG ANTARCTIC THAW
    An article in the Journal Science says that the melting of the West Antarctic Ice Sheet is a natural event not related to global warming contrary to claims by climate scientists. The WAIS is indeed melting quite rapidly receding at the rate of 400 feet per year but it has been doing so for thousands of years long before human activity and greenhouse gas emissions, having receded 800 miles since the last ice age.
23. 1999, WARM ARCTIC MAY ENHANCE GLOBAL WARMING. A sophisticated computer simulation model shows that increasing the temperature or snowfall on the Arctic tundra can triple its CO2 emissions from the soil of the tundra. The Arctic contains 1/3 of the earth’s soil stored carbon dioxide. The computer model shows a positive feedback look that can cause global warming to snowball because warming in itself can increase carbon dioxide in the air and accelerate the rate of warming.It is a frightening scenario that could cause global warming catastrophe to occur sooner than previously thought.

WHAT SCIENTISTS KNEW FOR SURE IN 2001

Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 5C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet.

WHAT SCIENTISTS KNEW FOR SURE IN 2007

Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 4C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet.

WHAT SCIENTISTS KNEW FOR SURE IN 2013

Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 3C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet.

WHAT SCIENTISTS KNEW FOR SURE IN 2015

Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 2C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet.

WHAT SCIENTISTS KNEW FOR SURE IN 2018

Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 1.5C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet.

WHAT SCIENTISTS KNOW FOR SURE IN 2020:Insights from Earth system model initial-condition large ensembles and future prospects, C. Deser, F. Lehner, K. B. Rodgers, T. Ault, T. L. Delworth, P. N. DiNezio, A. Fiore, C. Frankignoul, J. C. Fyfe, D. E. Horton, J. E. Kay, R. Knutti, N. S. Lovenduski, J. Marotzke, K. A. McKinnon, S. Minobe, J. Randerson, J. A. Screen, I. R. Simpson & M. Ting: Nature Climate Change (2020). Abstract: Internal variability in the climate system confounds assessment of human-induced climate change and imposes irreducible limits on the accuracy of climate change projections, especially at regional and decadal scales. A new collection of initial-condition large ensembles (LEs) generated with seven Earth system models under historical and future radiative forcing scenarios provides new insights into uncertainties due to internal variability versus model differences. These data enhance the assessment of climate change risks, including extreme events, and offer a powerful testbed for new methodologies aimed at separating forced signals from internal variability in the observational record. Opportunities and challenges confronting the design and dissemination of future LEs, including increased spatial resolution and model complexity alongside emerging Earth system applications, are discussed.