USA Government: Most of the warming of the past half-century is due to human activities

According to the U.S. Global Change Research Program*, "it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence."

Excerpt from 2017 report, Executive Summary:

    The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse gases (especially carbon dioxide) emitted globally. Without major reductions in emissions, the increase in annual average global temperature relative to preindustrial times could reach 9°F (5°C) or more by the end of this century. With significant reductions in emissions, the increase in annual average global temperature could be limited to 3.6°F (2°C) or less.
    The global atmospheric carbon dioxide (CO2) concentration has now passed 400 parts per million (ppm), a level that last occurred about 3 million years ago, when both global average temperature and sea level were significantly higher than today. Continued growth in CO2 emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens to hundreds of millions of years. There is broad consensus that the further and the faster the Earth system is pushed towards warming, the greater the risk of unanticipated changes and impacts, some of which are potentially large and irreversible.(Source)

Excerpt from 2014 report:
Multiple System Failures During Extreme Events
    Impacts are particularly severe when critical systems simultaneously fail. We have already seen multiple system failures during an extreme weather event in the United States, as when Hurricane Katrina struck New Orleans. Infrastructure and evacuation failures and collapse of critical response services during a storm is one example of multiple system failures. Another example is a loss of electrical power during heat waves or wildfires, which can reduce food and water safety. Air conditioning has helped reduce illness and death due to extreme heat, but if power is lost, everyone is vulnerable. By their nature, such events can exceed our capacity to respond. In succession, these events severely deplete resources needed to respond, from the individual to the national scale, but disproportionately affect the most vulnerable populations. (Source)

(The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future. A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.)

* 2014 National Climate Assessment. U.S. Global Change Research Program
1800 G Street, NW, Suite 9100, Washington, D.C. 20006 USA

References:
https://science2017.globalchange.gov/chapter/executive-summary/
http://nca2014.globalchange.gov/highlights/report-findings/widespread-im...


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"The Progress Trap - and how to avoid it" Copyright Daniel O'Leary, registered at
the Copyright Office, Consumer and Corporate Affairs, Canada on April 5, 1991 (ref 405917)