The industrial era is when most of the technologies that we see today got invented. In the process of creating and deploying these technologies, we have had to burn a lot of fossil fuels, destroy a lot of forests, pour a lot of fertilizers and irrigate a lot of land. Climate change is a consequence of all that activity. During the ice ages, the global surface temperature changes were caused by slight wobbles in the Earth's orbit around the sun. These wobbles result in slightly different amounts of solar energy reaching the Earth as the orbit changes in the Milankovitch cycles. When expressed in terms of “radiative forcing,” which is the average solar energy that is absorbed per square meter of the Earth's surface per second, the variation due to Milankovitch cycles is on the order of 0.5 Watts/m2. That may not seem like much, just 0.5% of the energy expended in a 100-Watt light bulb. When integrated over the entire surface of the Earth and over thousands of years, it is the difference between hot summer days versus a mile of ice on top of Chicago! In contrast, the greenhouse gases that humans have emitted to date are trapping an extra 3 Watts/m2 of solar energy, which is six times larger than the forcing due to the Milankovitch cycles! The Earth's climate system is now beginning to react to this human induced forcing. As a result, the rate of increase of global surface temperatures today is also an order of magnitude faster than those due to the Milankovitch cycles.
If global surface temperature changes happen slowly, or for that matter, if changes in any of the planet's other environmental characteristics happen slowly, the Earth's biosphere is able to gradually evolve and adapt to them. As part of the Earth's natural healing process, the fauna migrate to adapt to the changes and then the flora follow more slowly as the old forests give way to the new. But if surface temperature changes occur quickly as they are doing now, it's as if the Earth has developed a fever since the biosphere hasn't had time to adapt. At present, the average global surface temperature on Earth has increased by about 1.0 deg C (1.8 deg F) since pre-industrial times, which is as if the Earth has developed a 38 deg C (100 deg F) fever, in human terms. Now, a 38 deg C (100 deg F) fever is not too debilitating, though the patient could probably use an aspirin to mitigate the fever. But if the fever increases to 39 deg C (102 deg F) or higher, then more drastic measures would be called for. Similarly, climate scientists have estimated that if the Earth's average global surface temperature increases more than 2 deg C (3.6 deg F) from pre-industrial levels, then other climate feedback loops would kick in causing the surface temperature of the Earth to spiral out of control. Indeed, these days, even the 2 deg C increase is being seen as too dangerous and perhaps we must limit the increase to less than 1.5 deg C. But our current and future actions will decide whether the Earth's fever reaches such dangerous levels, for this fever is primarily caused by human activities on Earth.
But there's more to this story than just the fever. If the patient also has a coconut-sized growth by the side of his head, then a competent doctor ought to be treating that growth as well. Scientists at the Global Footprint Network have distilled human impact into a single figure, the ecological footprint, which they then compare with the available biological capacity of the planet. According to their calculations, the human ecological footprint has been exceeding the available biological capacity of the Earth since the early 1970s and it is currently in excess of that capacity by nearly 60%. In effect, we have been eating into the ecological capital of the planet for the past four decades. Clearly, this is unsustainable, meaning that this imbalance will end at some point in time. We can let this rebalancing happen abruptly as the Earth's bio-geophysical systems break down catastrophically in response to our continued demands on the Earth and collapse our industrial civilization. Or we can voluntarily reduce our per capita ecological footprint and constrain it to be lower than the biological capacity of the planet as we transition towards a steady-state mode of living.
Again, our future actions will decide how the Earth evolves.
Systems scientists at MIT created elaborate models in the 1970s to study the interplay between human economic systems and the planet's bio-geophysical systems and thereby predict when the current trajectory of the global industrial civilization reaches a breaking point, before which the transition to a steady-state civilization must ideally happen. Their models tracked six variables:
1) non-renewable resources remaining;
2) food per capita;
3) services per capita;
4) industrial output per capita;
5) global pollution; and
6) human population.
When they published the results of their simulations, including the predicted trajectories of these variables, in the book, Limits to Growth, in 1972, it became the best selling environmental book of all time, selling over 20 million copies worldwide! Their simulations predicted that human systems will reach a breaking point around 2025 in our present course leading to a civilizational collapse, but their models and simulated results were not taken seriously by the economists of that era . But in 2010, Dr. Graham Turner, a scientist working at the University of Melbourne in Australia, compared the 1972 predictions in Limits to Growth with the actual data from 1970 through 2000 and found an almost exact match between them for all six of the variables. Therefore, it is likely that a massive transformation of the global industrial civilization to a steady state mode of living will need to occur within the next decade or so before the predicted system collapse.
In the MIT systems model, the predicted breakdown in human economic systems occurs because the Earth's remaining non-renewable resources gets depleted over time. But that's a simplified, single variable stand-in for all of the Earth's complex bio-geophysical processes. Lately, in 2009, a group of 28 leading Earth systems and environmental scientists from different fields of expertise gathered to consider the various planetary bio-geophysical cycles and assess which ones are being stressed by human systems beyond tolerable limits. They proposed a framework of planetary boundaries defining a safe operating space for human systems to work within. Of the nine planetary life support systems that they considered, they quantified limits on seven of them and estimated that three of the boundaries have been crossed already. Here are the nine Earth's life support systems and the scientists' findings on them, ranked in the order of the most violated limit to the least:
1. Loss of Species: Currently, upwards of 100 species are going extinct per million species extant each year. The safe operating limit for this variable is estimated to be 10 species/million/year and therefore, human systems are causing tenfold as much extinction of species as is safely tolerable. However, some biologists have claimed that even the proposed extinction safe limit of 10 species/million/year is far too negligent since the background rate is less than 0.1 species/million/year. But suffice it to say that the current rate of extinction is orders of magnitude above acceptable limits for us to sustain.
2. BioGeoChemical Processes: Humans are extracting over 120 million tons of Nitrogen from the atmosphere each year, thereby interfering with the planetary nitrogen cycle. The scientists estimated that the safe limit is around 35 million tons of Nitrogen extracted each year, which means that the human extraction is 3.5 times the safe limit. Of the other biogeochemical process, the anthropogenic phosphorous that is being pumped into the ocean, the scientists found that the human contribution of 9 million tons per year is still below the safe limit of 11 million tons per year. Prior to the industrial revolution, human activities had no impact on both these biogeochemical processes.
3. Climate Change: The limit for climate change was estimated in two ways, a) by using the atmospheric CO2 (Carbon Dioxide) concentration and b) by using the anthropogenic radiative forcing. In terms of CO2 concentration, the scientists reiterated Dr. Jim Hansen's famous limit of 350 ppm (parts per million), which is the limit that spawned Bill McKibben’s global environmental organization, 350.org. At present, atmospheric CO2 concentrations are over 400 ppm, which is higher than the limit and violates it by about 50ppm. With respect to pre-industrial atmospheric CO2 levels of 280ppm, the current level is 1.7 times the allowed safe limit of excursion.
This limit is expressed in terms of just the atmospheric CO2 limit, even though human systems emit a lot of different greenhouse gases such as methane, nitrous oxide, black carbon, etc., and not just CO2. However, human systems also emit a number of aerosols, which function to cancel the effect of greenhouse gases and cool the Earth's surface. It is just an accident of our global emissions profile that the net effect of the other short-lived greenhouse gases such as methane, nitrous oxide, etc., are roughly equal and opposite to the effect of all the aerosols that we emit, mostly as a byproduct of our fossil fuel burning. Therefore, the atmospheric CO2 limit by itself serves as a proxy for the overall contribution of human activities, for the time being.
Scientists estimated the alternative radiative forcing limit to be 1.0 Watt/m2. While greenhouse gases emitted through human activities contribute 3.0 Watts/m2 of radiative forcing, it is estimated that the aerosols negate about 1.5 Watts/m2 of that forcing. Therefore, the net human induced radiative forcing is about 1.5 Watts/m2 which violates the prescribed limit of 1.0 Watt/m2 by 50%.
4. Ocean Acidification: Almost a quarter of the anthropogenic CO2 emissions is being absorbed in the ocean where the CO2 combines with water to become carbonic acid. As a result, the overall acidity of the ocean increases and this especially affects mollusks, clams and other shelled creatures that find it more difficult to form their shells. However, the scientists estimated that the safe limit for ocean acidity has not yet been breached though the increase in acidity since pre-industrial times is at 80% of the safe limit.
5. Land Use: The scientists estimated that the safe percentage limit of the Earth's land that can be converted to cropland is 15%, while the actual conversion that has occurred is 75% of the safe limit.
6. Fresh Water use: The scientists estimated that the safe limit for the total amount of fresh water that can be used by humans is 4000 km3/yr, while the current usage is 62% of the safe limit.
7. Ozone Depletion: The scientists estimated that the safe limit for ozone depletion has not yet been breached and the human activities have resulted in ozone depletion that is at 50% of the safe limit.
8. Atmospheric Aerosol Loading: Currently, about half of the radiative forcing due to anthropogenic greenhouse gases is being masked by the human emissions of aerosols such as SO2 (sulphur dioxide). However, aerosols cause respiratory problems in humans and other animals, and are estimated to be responsible for about 800,000 premature human deaths annually. Aerosols also cause acid rain, affect the monsoons and global circulation systems. However, the scientists could not estimate a safe limit for the human induced aerosol loading of the atmosphere.
9. Chemical Pollution: The safe limits on the chemical pollution of the Earth's land, air and sea due to human industrial activities have not yet been quantified. Chemical compounds such as insecticides, pesticides, PCBs, dioxins and other so-called Persistent Organic Pollutants (POPs) bioaccumulate in living beings leading to various disorders and cancers. Other pollutants such as herbicides, excreted pharmaceuticals, heavy metals and radionuclides have potentially irreversible and harmful effects on many biological organisms, including humans.
 A calculation of the radiative forcing due to the Milankovitch cycles is found here: http://www.skepticalscience.com/Milankovitch.html
 Here’s a discussion on why two degrees is an crucial number: http://www.pbs.org/newshour/bb/why-2-degrees-celsius-is-climate-changes-magic-number/
 The Global Footprint Network calculates an ecological overshoot day here: http://www.overshootday.org/
 A PDF scan of the original 1972 book, Limits to Growth, can be found here: http://www.donellameadows.org/wp-content/userfiles/Limits-to-Growth-digital-scan-version.pdf
 Economists such as Julian Simon argued that human ingenuity will overcome any scarcities as it is the ultimate resource. His 1981 book “Ultimate Resource” is now in its second version: Simon, Julian, The Ultimate Resource 2, Princeton University Press, Oct 1996, ISBN-13: 978-0691042695, http://amzn.to/1sw7gpC
 Graham Turner’s article in the Guardian can be accessed here: http://bit.ly/1tVayzy
 J. Rockstrom, et al., “Planetary Boundaries: Exploring the Safe Operating Space for Humanity,” vol 14, no 2, art 32, 2009. http://www.ecologyandsociety.org/vol14/iss2/art32/
 Hansen, J., et al, “Target atmospheric CO2: Where should humanity aim?,” Open Atmos. Sci. J. (2008), vol. 2, pp. 217-231, 2008. http://arxiv.org/abs/0804.1126
 Annual mortality due to atmospheric pollution is broken down here: http://www.who.int/mediacentre/factsheets/fs313/en/