Is recent climate change natural?

The warming seen over the last century has both natural and human sources, and scientists can tell them apart. The sun, for example, cannot explain the warming since the beginning of the 20th century. Other natural factors, including ocean currents, volcanoes, and “bounce-back” warming from the last ice age have also been ruled out.
Experiments and observations find that only human emissions can explain the amount of warming seen over the past few decades.(1),(2)

While warming has recently slowed from temporary natural processes, climate change has not stopped as some misleading graphs imply.(3) Scientists continue to see undeniable signs of change. For example, temperatures are still rising and the ocean’s chemistry is changing.(4),(5)

Solar Changes Don’t Significantly Affect the Climate
Since differences in sunlight drive the seasons, some people wonder if it also causes climate change. Atmospheric observations do not support sun-driven climate change. If the sun caused warming, all layers of the atmosphere would be warmer. Instead, satellites find the lower atmosphere (where greenhouse gases reside) has warmed significantly compared to the upper atmosphere.(6)

Solar activity
The sun’s brightness pulses roughly every 11 years when magnetic storms develop on its surface. These storms are called sunspots, which cause the sun to shine brighter than it would normally.(7),(8) Sunspots do not explain the sustained rise in temperatures over the past century since they fluctuate regularly.(9) Scientists monitor a small long-term increase in solar output unrelated to sunspots. They find this increase can only explain up to 10% of the warming in the 20th century.(10),(11)

temperature_change-01

Volcanoes Are Not Responsible for Climate Change
Since volcanoes erupt with massive amounts of heat, one might think volcanic eruptions explain Earth’s rising temperatures. Counter-intuitively, volcanoes actually have a short-term cooling effect.(12) According to NASA and USGS, volcanic eruptions disperse small particles that reflect incoming sunlight, causing the planet to temporarily cool.(13),(14) Over the past century, the largest volcanic eruptions lowered temperatures for a few years.(15)

volcanic-eruptions-temperatures-01

Source: NASA

While volcanoes do release carbon dioxide into the atmosphere, it is much less than human emissions. On average, volcanoes worldwide contribute only 1 percent the amount that humans do each year.(16),(17)

Climate Change Is Not Due to an Ice Age Recovery
You may have heard that the climate is warming because the Earth is bouncing back from an ice age – but that’s not exactly true. The Earth does warm during “interglacial periods”, the warm times between ice ages, but recent warming is occurring much faster than in the past.(18),(19)

Interglacial warming on overdrive
The Earth regularly cycles through ice ages and warm periods caused by changes in the air, ocean and orbit.(20 Ice ages typically last 100,000 years and the following warm times last about 10,000 years. We’re currently at the end of an unusually long warm period that began 12,000 years ago.(21)

observed-warming-01

Source: NASA and MIT

It is logical that the Earth is warming because we are in an interglacial period, but we’re warming much faster than previous “recovery” periods. During interglacial periods, global temperatures rise very slowly, about 0.2 degrees Fahrenheit per century. Over the past century the Earth’s surface has increased 1.4 degrees, multiple times the average interglacial rate.(22) NASA projects the predicted rate of warming for the next century is at least 20 times faster than the usual natural cycle.(23)

Milankovitch Cycles
The Earth’s orbit and axis regularly change over tens of thousands to hundreds of thousands of years. These processes are known as Milankovitch Cycles.(24) Although Milankovitch Cycles were a main cause of climatic changes in the past, they are much too slow to explain the recent warming observed over the past 100 years.(25),(26)

El Niño and La Niña do not explain climate change
El Niño and La Niña – changes in ocean currents that can affect weather globally – cause some of the Earth’s normal temperature variability.(27) In general, El Niño years tend to be warmer than average and La Niña years cooler.(28)

During El Niño events, the surface of the Pacific Ocean is warmer than normal. During La Niña events, the Pacific Ocean is the opposite. Although La Niña and El Niño are a regional occurrence, they have weather implications for many parts of the U.S. and the world.(29),(30),(31)

Neither are the cause of long-term changes. Warming from climate change has been documented for over a century, well outside the year-to-year variability from El Niño and La Niña.

elnino-effects-01

Source: Met Office (the UK’s national weather service)

 

 

SOURCES

  1. NASA: Climate Causes
  2. NASA Earth Observatory: Global Warming
  3. NASA: Ups and Downs of Climate Change
  4. ClearPath: World Temperatures Rising
  5. ClearPath: Ocean Acidification
  6. Lawrence Livermore National Laboratory: Study Finds Human Activity Affects Vertical Structure of Atmospheric Temperature
  7. NASA: Sunspots and the Solar Max
  8. University Corporation for Atmospheric Research
  9. NASA Earth Observatory: Global Warming
  10. NASA: Climate Causes
  11. American Geophysical Union
  12. United State Geological Survey: Volcanic Gases and Climate Change Overview
  13. NASA Earth Observatory: Global Warming
  14. NASA: Volcanoes and Climate Change
  15. Lawrence Livermore National Laboratory: Volcanoes Contribute to Recent Warming Hiatus
  16. NASA Earth Observatory: Global Warming
  17. NASA: How is Today’s Warming Different from the Past?
  18. NASA: Climate Q&A
  19. NOAA: Global Warming FAQ
  20. Utah Geological Survey
  21. NOAA: La Nina FAQ
  22. NASA: Climate Q&A
  23. NASA Earth Observatory: Global Warming
  24. NOAA: Astronomical Theory of Climate Change
  25. University of California, San Diego: Milankovitch Theory Supported
  26. NASA: Milutin Milankovitch
  27. Scripps Institute of Oceanography
  28. NOAA: La Nina FAQ
  29. NASA: La Niña
  30. National Geographic: La Nina
  31. NOAA: Can We Blame El Nino?

GRAPHIC SOURCES

  1. Natural Warming – Climate Change via AGCI
  2. Sun – Lean, et. al via NASA
  3. Volcanoes – Lean, et. al via NASA
  4. Rates of warming – NASA: Climate Q&A, NASA: Decadal Temp, MIT
%d bloggers like this: