Pro:
The Intergovernmental Panel on Climate Change (IPCC) stated in its Summary for Policymakers in 2013:
‘Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased’
https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_SPM_FINAL.pdf
‘Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century’
https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_SPM_FINAL.pdf
Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850 (see Figure SPM.1). In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years (medium confidence). {2.4, 5.3}
https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_SPM_FINAL.pdf
And From NASA
Earth’s global average surface temperature in 2020 tied with 2016 as the warmest year on record, according to an analysis by NASA.
Continuing the planet’s long-term warming trend, the year’s globally averaged temperature was 1.84 degrees Fahrenheit (1.02 degrees Celsius) warmer than the baseline 1951-1980 mean, according to scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York. 2020 edged out 2016 by a very small amount, within the margin of error of the analysis, making the years effectively tied for the warmest year on record.
https://climate.nasa.gov/climate_resources/139/video-global-warming-from-1880-to-2020/
Nineteen of the warmest years have occurred since 2000, with the exception of 1998. The year 2020 tied with 2016 for the warmest year on record since record-keeping began in 1880 (source: NASA/GISS). This research is broadly consistent with similar constructions prepared by the Climatic Research Unit and the National Oceanic and Atmospheric Administration.
https://climate.nasa.gov/vital-signs/global-temperature/
And from the Climatic Research Unit in the UK
The time series shows the combined global land and marine surface temperature record from 1850 to 2020. Last year (2020) was the second warmest on record using our latest analysis, referred to as HadCRUT5 Analysis (Morice et al., 2021). Each decade since the 1980s has been warmer than all preceding decades in our record. The average global temperature during the 2010s (2011-2020) was 0.76(±0.05) °C above the 1961-1990 average and 1.12(±0.11) °C above the late nineteenth century average. The Earth’s surface has warmed by about 0.2 °C per decade since the 1970s. Not surprisingly, the last six years are the warmest six years in our record.
https://sites.uea.ac.uk/documents/421974/1295957/CRU-Info-sheet-2021.pdf/4e8e6be5-2b01-44ff-a139-62e4ec79f09f
Con:
New studies sponsored by the Smithsonian Institution show that Earth experienced much warmer periods in the past than we have today. In the recent past global mean temperature appears to have risen by as much as 9-14°F (5-8°C) to an average temperature as high as 73°F compared to the 20th century global average of 56.7°F.
Even further in the past global temperatures repeatedly rose above 80°F and even 90°F—temperatures too warm for polar ice caps or Greenland ice sheets. All of this happened naturally – Earth’s hottest periods occurred before humans existed.
One of the most common arguments by climate alarmists is that our current global temperature is the “hottest ever” and humans burning fossil fuels are to blame. But, results from a Smithsonian Institution project examining Earth’s average surface temperature over the past 500 million years showed that for most of the time, global temperatures appear to have been too warm for polar ice caps.
Modern human civilization, with its permanent agriculture and settlements, has developed over just the past 10,000 years. The period has generally been one of low temperatures and relative global climate stability. Compared to most of Earth’s history, today is unusually cold; we now live in what geologists call an interglacial—a period between glaciations of an ice age where ice sheets up to a mile thick covered what is now New York and Chicago.
As seen in Figure 1, just over 55 million years ago, the global temperature appears to have risen by as much as 5-8°C (9-14°F) to an average temperature as high as 73°F (22.8°C). (Today’s 20th century global average is 56.7°F or 13.7°C.) At about the same time, data from fossils and ocean sediments record a massive release of carbon dioxide into the atmosphere, at least doubling or possibly even quadrupling the background concentrations. What is unclear is which came first; the temperature rise or the carbon dioxide.
Figure 1. Results from a Smithsonian Institution project showing Earth’s average surface temperature over the past 500 million years. For most of the time, global temperatures appear to have been too warm (red portions of line) for persistent polar ice caps. The most recent 50 million years are an exception. Image adapted from Smithsonian National Museum of Natural History and published on climate.gov
This all occurred naturally without human intervention, yet today it is said that humans alone are driving climate change by burning fossil fuels that release carbon dioxide into the atmosphere. Alarm is being raised over a global temperature gain of just 1.5°C, a study published in March 2013 concluded that global average temperature is now higher than it has been for most of the last 11,300 years, but it is miniscule to the natural temperature increases Earth has seen in the past.
References:
- What’s the hottest Earth’s ever been? NOAA’s climate.gov, January 18th 2020: https://www.climate.gov/news-features/climate-qa/whats-hottest-earths-ever-been
- A Reconstruction of Regional and Global Temperature for the Past 11,300 Years, Science 08 Mar 2013: DOI: 10.1126/science.1228026 https://science.sciencemag.org/content/339/6124/1198
- A 500-million-year survey of Earth’s climate reveals dire warning for humanity, Science Magazine, May. 22, 2019: https://www.sciencemag.org/news/2019/05/500-million-year-survey-earths-climate-reveals-dire-warning-humanity