Climate Change Caused by Humans
Summary of Key Points

A summary of the key points made in the collection of Web pages at this site is listed below:

The Scientific Consensus:

  1. Climate change has been extensively researched and the overwhelming majority of climate scientists agree that the observed modern day global warming is unprecedented and is very likely caused by humans.
  2. Although there is little serious debate between climate experts, many in the general public still think that these scientists are unsure about climate change and the role that humans have played in modern day global warming.
  3. 96.2% of climatologists who are active in climate research believe that mean global temperatures have risen compared to pre-1800s levels.
  4. 97.4% believe that human activity is a significant factor in changing mean global temperatures.
  5. IPCC WGI 4th Assessment Report leaves little doubt that the present climate is experiencing an unprecedented global warming rate which is primarily due to human activities.
  6. No scientific body of national or international standing holds a dissenting opinion from IPCC 2007.
  7. Recent observations show that greenhouse gas emissions and many aspects of the climate are changing near the upper boundary of the IPCC range of projections.
  8. Many key climate indicators are already moving beyond the patterns of natural variability within which contemporary society and economy have developed and thrived. These indicators include global mean surface temperature, sea-level rise, global ocean temperature, Arctic sea ice extent, ocean acidification, and extreme climatic events.
  9. With unabated emissions, many trends in climate will likely accelerate, leading to an increasing risk of abrupt or irreversible climatic shifts.

Determining the Climate Record:

  1. Scientists can use proxy data to determine past climate before thermometers were used. These proxy data include: ice cores, tree rings, pollen, boreholes, corals and marine sediment, lake sediment, and historical documents.
  2. A careful analysis of these multiple proxy data makes it quite clear that the modern climate is much warmer than at any time in the past 1800 years.

Impact of Greenhouse Gases:

  1. Increasing greenhouse gases causes an increase in atmospheric temperature.
  2. Although greenhouse gas concentrations appear to be small (less than one percent), their effect is certainly not.
  3. The greenhouse effect from natural greenhouse gas concentrations prior to the Industrial Revolution has kept the Earth's surface about 33 oC warmer than with an atmosphere with no greenhouse gases.
  4. CO2 concentrations have cycled between high (300 ppm) and low (190 ppm) values with a period of approximately 100,000 years and the climate has closely followed this cycle - high levels of CO2 occur in warmer periods and low levels of CO2 occur in cooler periods.
  5. Present day values of CO2 have far exceeded this range and have done so at an extraordinarily fast rate.
  6. In the past 250 years, 1,200 billion tons of CO2 have been released to the atmosphere. Most of this is from fossil fuel emissions and half of these emissions has occurred only since the mid-1970s.
  7. The growth rate of CO2 doubles about every 30 years.
  8. By 2050, assuming a do-nothing approach, CO2 concentrations will reach 560 ppm, doubling the 280 ppm pre-Industrial Revolution value.
  9. Long-lived greenhouse gases have contributed the majority of warming since 1750.
  10. Satellite measurements show that outgoing long-wave (LW) radiation is being trapped in the atmosphere by greenhouse gases.
  11. Downwelling LW at the surface has been measured and is increasing due to greenhouse gases.
  12. This experimental data effectively ends the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming.
  13. Despite the large uncertainty in the magnitude of cloud feedbacks, the overall picture of feedbacks in a warmer world is one that is positive - meaning that greenhouse gas warming will be enhanced by these mechanisms.
  14. Climate sensitivity is the term used to describe the equilibrium global surface air temperature change due to a doubling of CO2 from 280 ppm (pre-Industrial Revolution) to 560 ppm.
  15. Various observations show a climate sensitivity value of about 3oC, with a likely range of about 2 - 4.5oC.
  16. The lower value of climate sensitivity of 2oC is fairly well constrained which means that if emissions are not stabilized very soon, significant global warming is inevitable.
  17. Earth could become ice-free even with the current levels of CO2.
  18. The last time Earth was ice-free, sea levels were 120m (~400 feet) higher.
  19. A warmer troposphere and a cooler stratosphere have been observed. This can only happen if greenhouse gases are trapping outgoing heat from the troposphere.

The Smoking Gun for Humans:

  1. Isotopic analysis of carbon shows that the source of increasing carbon in the atmosphere is primarily from human emissions.
  2. There are no other known plausible arguments to explain modern carbon levels without humans being the main cause of the increase.
  3. About half of the emitted carbon by humans is taken up in the oceans and in plants.
  4. When climate models consider only natural causes of climate change, the actual recorded climate cannot be predicted.
  5. When human forcing is added to natural forcing observed climate across the globe is accurately modeled.
  6. The climate observed today could NOT have happened if humans did not exist - it is impossible for natural forces to have caused today's climate.
  7. Tropospheric warming with stratospheric cooling is essentially another "smoking gun" for anthropogenic global warming.
  8. Solar forcing, cloud cover, ENSO, PDO, NAO, etc. cannot explain a cooler stratosphere. Increasing greenhouse gases explain this coupling very well and climate models predict a warmer troposphere and a cooler stratosphere with increased greenhouse gases.

Natural Causes of Climate Change:

  1. Carbon is cycled into and out of the atmosphere by plate tectonics over millions of years so it cannot be responsible for modern day rapid global warming.
  2. Milankovitch Cycles (orbital forcing) cause climate to change over tens to hundreds of thousand years so it cannot be responsible for modern day rapid global warming.
  3. The Pacific Decadal Oscillation (PDO) cannot explain the long term warming trend observed in the modern climate record.
  4. Current estimates suggest that only 0.1 oC of the 0.8 oC of warming since the late 1800s is due to solar irradiance.
  5. Since direct satellite measurements (1980 - present) solar contribution to the observed rapid warming is negligible.
  6. There is no evidence that variations in the strength of the sun are the cause of the modern day rapid global warming.
  7. Because volcanic eruptions and El Nino events cause climate change on a scale of one to three years, these natural forcing mechanisms cannot be responsible for modern day rapid global warming.

Global Cooling?:

  1. Solar dimming from pollution blocks incoming sunlight and leads to a cooling effect on the climate.
  2. Solar dimming was effective in masking greenhouse warming, but only up to the 1980s, when dimming gradually transformed into brightening.
  3. Clean air legislation in the U.S. and other countries in the 1970s resulted in a reduced amount of global dimming pollution.
  4. Since then, the uncovered greenhouse effect has revealed its full dimension, as manifested in a rapid temperature rise (+0.38oC/decade over land since mid-1980s).
  5. Despite numerous claims that global warming ended in 1998, 2001, or 2003, the data shows these claims are groundless.
  6. 19 of the warmest years on record have occurred in the past 25 years. The warmest years globally have been 1998 and 2005 with the years 2002, 2007, and 2003 close behind. The warmest decade has been the last ten years and the warming has been widespread globally.
  7. If one "cherry-picks" a small subset of data, it may appear on the surface to show global cooling. However, if one looks at the long-term trends it is quite obvious that global temperatures have been increasing since 1880 and at a faster rate in the past two decades.
  8. A change in the heat stored in the ocean is a better indicator of climate change than changes in atmospheric heat. The oceans have warmed significantly in recent years and the trend is 50% greater than that reported by the IPCC in 2007. There is certainly no global cooling in the oceans!

Climate Models & Accuracy

  1. Climate models are based upon well-established laws of physics and use a wealth of actual observations.
  2. Models are able to reproduce observed features of current climate and past climate changes. Confidence in model estimates is higher for some climate variables (e.g., temperature) than for others (e.g., precipitation).
  3. Over several decades of development, models have consistently provided a robust and unambiguous picture of significant climate warming in response to increasing greenhouse gases.
  4. Climate models provide credible quantitative estimates of future climate change, particularly at continental scales and above.
  5. Predicting climate is easier than predicting weather. A person who claims "how can we predict climate change over decades when we can't even predict tomorrow's weather?" has a fundamental misunderstanding of modeling.

Modern Day Climate Change

  1. 20 of the warmest years on record have occurred in the past 25 years. The warmest years globally were 2010 and 2005 with the years 2009, 2007, 2006, 2003, 2002, and 1998 all tied for 2nd within statistical certainty. The warmest decade has been the 2000s, and each of the past three decades has been warmer than the decade before and each set records at their end. The odds of this being a natural occurrence are estimated to be one in a billion!
  2. Temperature data from 1850 to present shows that there has been an increasing trend and the rate of warming has increased in the past few decades.
  3. Surface temperatures north of latitude 60o are warming at an accelerated rate in the past few decades.
  4. The Arctic was experiencing long-term cooling in the past 2000 years according to Milankovitch cycles until very recently. The cooling trend was reversed during the 20th century, with four of the five warmest decades of the 2000-year-long reconstruction occurring between 1950 and 2000.
  5. Sea ice extent has been dramatically reduced since the 1950s.
  6. Since measurements began in 1953, there has been a dramatic decrease in sea ice thickness.
  7. Greenland is losing ice mass and the rate is accelerating.
  8. Antarctica is losing ice mass and the rate is accelerating.
  9. The average mass balance of the glaciers with available long-term observation series around the world continues to decrease.
  10. 90% of worldwide glaciers are retreating.
  11. Much of the heat that is delivered by the sun is stored in the Earth's oceans while only a fraction of this heat is stored in the atmosphere. Therefore, a change in the heat stored in the ocean is a better indicator of climate change than changes in atmospheric heat.
  12. The heat content of the oceans is increasing.
  13. The oceans are taking in almost all of the excess heat since the 1970s which underscores the point that ocean heat content is a better indicator of global warming than atmospheric temperatures.
  14. The Palmer Drought Severity Index (PDSI) curve reveals widespread increasing African drought, especially in the Sahel.
  15. Global warming due to human activities is increasing the severity of drought in areas that already have drought and causing more rainfall in areas that are already wet.
  16. According to the US Climate Extremes Index (CEI), extremes in climate are on the increase since 1970.
  17. The concentration of CO2 has reached a record high relative to more than the past 500,000 years and has done so at an exceptionally fast rate.
  18. Most of the warming in the past 50 years is attributable to human activities.
  19. Although large climate changes have occurred in the past, there is no evidence that they took place at a faster rate than the present warming.
  20. If projections of a 5 oC warming in this century are realized, Earth will have experienced the same amount of global warming as it did at the end of the last ice age.
  21. There is no evidence that this rate is matched to a comparable global temperature increase over the last 50 million years!
  22. Sea level gradually rose in the 20th century and is currently rising at an increased rate, after a period of little change between AD 0 and AD 1900. The trend is 50% greater than that reported by the IPCC in 2007.
  23. Sea level is predicted to rise at an even greater rate in this century, with 20th century estimates of 1.7 mm per year.
  24. When climate warms, ice on land melts and flows back into the oceans raising sea levels.
  25. When the oceans warm, the water expands (thermal expansion) which raises sea levels.
  26. IPCC 1990 projected sea level increases were too conservative. The latest observations show that sea levels have risen faster than previous projections.
  27. Greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2-11% by 2100.
  28. Substantial increases in the frequency of the most intense cyclones are expected with increases of the order of 20% in the precipitation rate within 100 km of the storm centre.
  29. Rising sea-levels will result in more damage from hurricanes even if hurricane strength remains unchanged.

The Global Warming Denial Machine

  1. There has been a concerted effort, especially by ExxonMobil to undermine the science of climate change and to confuse the general public.
  2. Many conservative think tanks have been set up to look like scientific think tanks when in reality they are fossil fuel-funded public relations organizations.
  3. The skeptics are supported by politically powerful CTTs funded by wealthy foundations and corporations.
  4. Some efforts to confuse the public about global warming include (among many others):
  5. All of the claims made in these propaganda stunts have been thoroughly debunked.
  6. There are no peer-reviewed articles that show a natural cause of climate change while also showing why greenhouse gases are not causing the majority of the observed global warming.
  7. With very few exceptions, skeptics of AGW are not actively publishing in climate-related peer-reviewed scholarly journals.

In February 2010, John Cook of Skeptical Science posted a brilliant summary of the multiple lines of evidence to support AGW. Check it out.

So we are left with three possible conclusions:

Common sense and a sense of probability should lead one to the likely correct choice (#1) above. The first person to show proof of what IS causing the modern day global warming and that it is not AGW is likely to be the next Nobel science winner.

Next: Suggested Reading

Many of the "contrarian arguments" one might run across are scientifically debunked at:
Skeptical Science: Examining Global Warming Skepticism
Coby Beck's: How to Talk to a Climate Sceptic's FAQ page: Responses to common contrarian arguments

Scott A. Mandia
Professor - Physical Sciences
T-202 Smithtown Sciences Bldg.
533 College Rd.
Selden, NY 11784
(631) 451-4104

Last Updated: 11/7/18