Correct. There is widespread retreat of the ice edge around all of the Greenland Ice Sheet. This is well documented.

I do not agree with this statement. Ice sheet edge retreat is not a direct measure of ice mass loss. Even if ice edge retreat happens in bursts, we cannot determine from that measure how much total ice will be lost each year. Also, sea level rise is the result of a combination of changes that include land ice melt, ocean heat causing ocean volume to increase, and local vertical land motion. I would argue that the current spread in future projections of sea level rise is not highly influenced by Greenland glacier retreat rates.

It's useful to note that sea levels will not rise the same everywhere. Some coastal locations will experience much more or less sea level rise than the global average. For example, the U.S. Gulf Coast will experience more sea level rise than the global average.

It is correct that there is no longer a reasonable scenario in which the Greenland Ice Sheet is fully stabilized or gains significant ice. However, this statement ignores the very important matter of how quickly ice is lost. An excellent recent study by Aschwanden et al ([(https://advances.sciencemag.org/content/5/6/eaav9396]) demonstrates what a dramatic difference there is in ice loss under a low emissions scenario v. a high emissions scenario. The critical matter right now (which has already been understood by glaciologists for some time) is not whether or not we lose ice from Greenland but how quickly ice is lost.

Scientists may quibble over this. Glaciologists usually divide Earth's ice into 3 categories: Greenland Ice Sheet, Antarctic Ice Sheet, all other glaciers/ice caps. The amount of ice lost annually from Greenland is now very similar to that lost each year from the 'all other glaciers/ice caps' category.

Yes, this is accurate. The Greenland Ice Sheet began consistently losing ice year-after-year around the turn of the 21st century and this number is close to the recent average for annual ice loss.

Measuring changes in gravity is one important method used to determine how much ice is lost (or gained). There actually does not need to be a large change in ice mass for it to create a measurable change in the gravitational field. So it is a bit misleading to suggest that 'measurable change in the gravitational field' is synonymous with 'massive' ice loss. It is not. But it IS true that Greenland has lost a lot of ice in recent years. In fact, 2012 and 2019 both set records for ice loss within the historical record.

This is subjective. But given the scenarios under consideration by the IPCC, they do not suggest that a long delay in action can create the same future result as more immediate action. This can be seen by considering any of the projections that compare RCP2.6, RCP4.5, and RCP8.5. For example, in the latest Special Report. With explanation of scenarios here.

There have been significant changes in drought, but it has included both increases and decreases. "There is medium confidence that since the 1950s some regions of the world have experienced a trend to more intense and longer droughts, in particular in southern Europe and West Africa, but in some regions droughts have become less frequent, less intense, or shorter, for example, in central North America and northwestern Australia." from IPCC

There is clear evidence that floods due to sea level rise have increased to the present, and are expected to increase more rapidly into the future. Details are available in the new IPCC Special Report on the Ocean and Cryosphere in a Changing Climate.

Unfortunately, the issue at hand is not isolated to additional CO2 in the atmosphere. Decision makers must address the full suite of changing that occur with climate change, including temperature and precipitation change, nutrient availability, etc. All crops will not respond the same to climate change, but overall global decreases in crop yield and reductions in food security are actually a major concern. This paper provides an example of research on the influence of climate change on crop yield.

The more important note is that CO2 is a greenhouse gas. Thus, it is able to change the atmosphere's ability to absorb solar radiation and increase temperatures.

In fact, the basic chemistry and physics of climate change and the greenhouse effect have been well understood for more than a century. Here are a few of those milestones:

1859 – John Tyndall discovers that some gases block infrared radiation. He suggests that changes in the concentration of the gases could bring climate change.

1896 – Svante Arrhenius publishes first calculation of global warming from human emissions of CO2: doubling the CO2 in the atmosphere would raise global temp some 5-6°C (9-11°F)

1897 – Thomas Chamberlin produces a model for global carbon exchange including feedbacks.

1938 – Guy Callendar argues that CO2 greenhouse global warming is underway, reviving interest in the question.

1960 – Charles Keeling accurately measures CO2 in the Earth's atmosphere and detects an annual rise. The level is 315 ppm. Suki Manabe and Richard Wetherald make a convincing calculation that doubling CO2 would raise world temperatures a couple of degrees.

1977 – Scientific opinion tends to converge on global warming, not cooling, as the chief climate risk in next century.

As noted, this is false. The role of humans is well established, and has been for a number of decades. An example of what the climate would look like without anthropogenic forcing is available in this IPCC figure.

It is accurate that the Earth's climate has experienced natural variability including warm and cold phases. However, the crux is that these changes have NOT happened with human populations at a level even remotely approaching today's population and development levels. The world population reached 1 billion in ~1800 (link) and climate within the 1000-800 years before present timescale was quite stable. Human populations, economies, infrastructure, development have formed during a period of climate stability.

Greenland is losing ice at the edges - both through direct melt and by calving off icebergs - and it is getting thicker in the middle. However, the amount of thickening in the middle is a fraction of the loss at the edges. Instead, the loss at the edges is vastly outpacing new snow accumulation. Ice loss from Greenland has accelerated over the last 3 decades and in the recent decade Greenland has lost about 300 gigatons of ice per year (almost 1 mm of sea level rise per year). This paper provides a good overview. Also, Greenland glacier ice is not 'rock solid'. Glacier ice flows (think of it as very slow moving rivers), and ice is constantly moving from the center of the ice sheet to the edge. Change has been rapid and large.

Here the author is muddling the issue of climate change by introducing time periods that are unrelated. Human-caused climate change via significant greenhouse gas emissions as discussed today only refers to the last roughly 100 years. This human activity has completely altered the Earth's climate from any natural trend that may occur over 1,000s of years. The "millenial" scale is not relevant to the discussion of current human-caused climate change.

This statement is misleading. Scientists do find evidence for warmer summers during a period roughly 5,000 years ago. However, these were also accompanied by colder winters in many places, and some places did not have warmer summers. The changes in climate at that time were the result of natural changes in Earth's orbit. However, Earth's orbit is not the cause of the warming over the last ~100 years.

There is no question that this is correct. The future rate of ice loss (from Antactica and elsewhere) is linked to the rate of global warming and that is determined primarily by human activity. A useful reference is: Clark, P. U. et al. (2016), Consequences of twenty-first century policy for multi-millennial climate and sea-level change, Nature Climate change, 6(4), 360–369, doi:10.1038/nclimate2923.

Following earlier theory work by Jeremy Bassis and others, the marine ice-cliff instability term was introduced (as much as I'm aware) in 2015 in this paper: Pollard, D., R. M. DeConto, and R. B. Alley (2015), Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure, Earth Planet Sc Lett, 412, 112–121, doi:10.1016/j.epsl.2014.12.035. The two lead authors then continued to investigate the mechanism using computer simulations, publishing a paper in Nature in 2016. The idea of the marine ice-cliff instability is still fairly controversial amongst glaciologists (NOT to be confused with the marine ice sheet instability, which is not controversial and is another reason that West Antarctica is vulnerable to rapid ice loss). A recent paper in Nature (Wise, M. G., J. A. Dowdeswell, M. Jakobsson, and R. D. Larter (2017), Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks, Nature, 550(7677), 506–510, doi:10.1038/nature24458.) suggests that there is observational evidence from ancient iceberg tracks, but not all glaciologists are convinced that this new data fully supports the marine ice-cliff instability. Whether the marine ice-cliff instability is something that is likely to occur in Antarctica is still debated amongst scientists.

Multiple studies have indicated that the marine ice sheet instability is likely already underway in the Pine Island region: Joughin, I., B. E. Smith, and B. Medley (2014), Marine Ice Sheet Collapse Potentially Under Way for the Thwaites Glacier Basin, West Antarctica, Science, 344(6185), 735–738, doi:10.1126/science.1249055. and Rignot, E., J. Mouginot, M. Morlighem, H. Seroussi, and B. Scheuchl (2014), Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011, Geophys Res Lett, doi:10.1002/(ISSN)1944-8007.

There has been no evidence to suggest that the loss of ice from this region will stop. However, that the loss of ice from marine ice sheet instability is underway does NOT mean that the entire West Antarctic Ice Sheet will destabilize this century. There is no evidence for the ice loss to occur that quickly.

As noted earlier, I think there is still some disagreement on whether these are conclusive results.

This was a common perspective before the era of satellite data that showed rapid changes on ice sheets.

I actually think that many scientists already felt that humans should be in "emergency mode". For reference, consider the "World Scientists' Warning to Humanity", first written in 1992 and reprised this year. But I agree that the possibility of marine ice-cliff instability raised another alarm bell.

Many people in the glaciology community are working to improve computer models of ice sheet change.

This timeline represented here is not an accurate representation of the paper findings. Figure 4 in the paper is the most helpful. It shows that 11 feet by 2100 in not expected in the tested worst case. However, the paper also addresses sea level rise past 2100. Under the Intergovernmental Panel on Climate Change Representative Concentration Pathway (RCP) 8.5 (the worst case they discussed), the paper suggests sea level rise from Antarctica alone could be 15.65 +/- 2 m by 2500. These numbers are shown in Figure 5 of the Pollard and DeConto Nature paper. A still deeply disturbing finding.

That is true. For a recent look at how our ideas of likely future sea level rise have changed, this is a useful short piece from Science: How high will the seas rise?. If marine ice-cliff instability is confirmed as a likely or expected method of Antarctic ice loss then ice loss can occur more quickly than previously thought.

It is difficult to say just what the author means here. Certainly, sea levels are already rising, already creating higher tides, coastal erosion, and forcing relocation of coastal people. Sea level rise is a constant and ongoing process, so with no numbers here for the quantity he is addressing it is hard to say whether this statement is fully accurate.

Yes. Research suggests that the ice sheet was likely near balance up until close to the turn of the 21st century.

See my earlier comment about the discussions still going on in the glaciology community regarding whether this feedback loop is likely to occur in the real world (it is clear that the physics is accurate, but whether we see the right conditions in the real world is still in debate).

There are many important science questions, but it is true that understanding the rate of ice loss at Pine Island and Thwaites Glaciers is very important for understanding how quickly future sea level rise will happen. Understanding Thwaites is arguable more important than Pine Island, though they are part of a connect region of ice. A recent peer-reviewed science article that takes a closer look at the question and explains the importance of understanding changes is: Scambos, T. A. et al. (2017), How much, how fast? A science review and outlook for research on the instability of Antarctica's Thwaites Glacier in the 21st century, Global Planet Change, 153, 16–34, doi:10.1016/j.gloplacha.2017.04.008.

Sea ice cover is also better at reflecting energy from the sun than open ocean. This makes sea ice an important player in keeping the ocean surface from taking up additional heat.

This is an important change to emphasize because sea ice thickness is also an indicator of sea ice age. While thin sea ice can reform more quickly year to year, thick sea ice requires more time. The loss of thick sea ice is perhaps even more striking than the loss of ice area.

I agree with the excellent comments and observations made by others on this article. That includes Tad Pfeffer's comment pointing out that it is well understood across the science community that the Greenland Ice Sheet is losing significant amounts of ice and this amount is likely to increase in the future.

Again, "ice sheet" is more accurate than "ice cap". In general, the term "ice cap" is not used in scientific literature. As a result, I think it's use is confusing in the media, too, as people have a hard time understanding if it refers to ice sheets (thick ice on land) or sea ice (which are very different!).

This is true. Perhaps more important, though, is that surface melt in Greenland started early this year and included several large spikes. While current surface melt is not currently larger than the record-breaking 2012 melt year, it is ahead of 2013-2015 so far. The NSIDC has an excellent summary of current melt status for Greenland here.

This is a reasonable number for the total mean sea level rise if all of the Greenland Ice Sheet melted. Total melt of the Greenland Ice Sheet, however, is essentially impossible over the next several hundred years. Nevertheless, the Greenland Ice Sheet has been losing large amounts of ice and the rate of loss has increased over the recent decade or so (references include this paper and this one). The amount of ice Greenland has lost to date has already raised global sea levels and projections indicate the possibility of 0.5 m more sea level rise from Greenland alone over the coming several centuries.

The issue of drinking water availability is more nuanced than either extreme represented here. Increases in glacier melt can have a wide range of impacts depending on the location and timing of the change. For example, complete loss of an alpine glacier that is used for summer drinking water can result in dramatic water shortages for those communities. On the other hand, increasing glacier melt can mean short term (decades) increases in river water - such as for the upper Indus basin noted in this study. Unfortunately, increased water availability from glacier loss is bound to end once those glaciers disappear.

Absolutely false.

This study examined Antarctic ice sheet mass changes until 2008 and presents a single study of Antarctic ice mass. Also, the mechanism indicated for mass change - increased snowfall - is actually a result of a warming atmosphere and an understood change that is consistent with scientific understanding. Perhaps more pertinent, however, is that other studies, including of more recent changes have suggested overall ice loss and, most important, large future ice loss. Recently published articles (Joughin et al. 2014, Rignot et al. 2014) looking at changes on coastal Antarctic ice shelves shows that we have likely already entered a period of major ice shelf retreat, with no mechanisms in sight to stop this retreat over the next 100s of years. The implication of ice shelf and glacier retreat around the Antarctic coast is increased ice loss over the next 10s to 1000s of years. There is a very large body of scientific work examining Antarctic ice sheet mass trends and there is community agreement that future ice loss is expected and will be worse if global temperatures increase more.

True.

True.

All reductions to greenhouse gas emissions impact associated climate changes. The question is the level of impact that is being sought and the amount of reductions needed to do that. Science can provide data on this question (that is largely the work of the IPCC), but policy, law, ethics, etc. must decide the course of action.

This need not be an open question. We can measure the Earth's temperature and it is clear that the Earth's atmosphere is warming.

Also, the problem of confusing weather (short-term change) with climate (long-term change).

See responses to similar statements by other candidates above.

Agreed.

Please reference any number of the reports, etc. that I've cited above. There is excellent support that global warming is real and human-caused.

Climate science is most certainly a science and based on establish scientific principles of observation, math, chemistry, physics, and other basic sciences. The idea that climate science is a religion is, frankly, offensive.

This is false. Heating of the atmosphere via greenhouse gases is a well-understood phenomena based on basic physics and chemistry. It is well-defined and certainly open to disproving. Unfortunately, the wealth of evidence from research suggests this will not happen.

This is true, with more information here.

There are some areas where there is more scientific certainty on the impacts and other areas where there is less. Table SPM.1 in this summary provide a look as some impacts and the scientific certainty of their occurrence.

Agree with VictorVenema. Human-caused climate change is proven to the extent than any science is proven.

True. This IPCC report covers many of the impacts.

See earlier reference to research establishing that more than 97% of published worked during 1991-2011 supports the position of human-caused climate change. (I actually think there's been an updated study on this looking at papers since 2011 but can't find the reference at the moment.)

As discussed in earlier comments, the science has clearly established the dominant cause of current climate change is man-made, with near 100% confidence. Given the complexity of climate system, seeking a percent attribution is unreasonable.

As discussed in earlier comments, the science has clearly established the dominant cause of current climate change is man-made, with near 100% confidence. Given the complexity of climate system, seeking a percent attribution is unreasonable.

True, as noted in comments for earlier candidates.

True, as noted above.

This is true. See references in the IPCC Sea Level chapter and Cryosphere chapter.

The amount of greenhouse gases emitted - and the associated increases in global temperature - will effect the amount of sea level rise experienced in the future. This is true both for the short-term (e.g., 100 years - see this paper) and the long-term (e.g., 2000 years - see this paper). Also reference figures 13.10 and 13.11 in this IPCC Chapter on Sea Level.

Scientist have understood that greenhouse gas emissions will warm the atmosphere since at least 1896, when Svante Arrhenius calculated the warming expected with a doubling of atmospheric carbon dioxide. Nevertheless, it is true that most research on climate change has occurred over the last several decades. I'd like to point out that this is not a short period of time to be researching a topic, however, and thousands of papers have addressed the issue from many different perspectives. Also, simply because the research has been conducted over the last several decades does not mean that we only understand that changes during that time. There are many proxies (for example, measuring air bubbles in ice cores) that can tell us about the state of the Earth's atmosphere and environment hundreds and even thousands of years ago.

VictorVenema addresses this as well as comments for Candidate 12.

Confusing weather and climate is a common problem and confusing to the public and others. Weather is the conditions we see day to day and even year to year. Climate is long-term (usually 20-30 year) trends. So our climate can be warming at the same time that you experience very cold weather. This animation provides a useful explanation of the difference between weather and climate.

Scientists have actually considered this question very closely. Along with the comment I provide earlier, it might be useful to review figure SPM.6 in this IPCC summary report, which shows that currently observed temperatures, sea ice, and ocean heat content cannot occur if only natural changes are at play.

It is almost difficult to know where to begin for this statement. As a starting point, a review of the IPCC Fifth Assessment would be in order. Also a reminder that the IPCC reports are supported by 195 current member countries and synthesize the work of thousands of scientists globally. These reports clearly document that human release of greenhouse gases is already causing heating of the Earth's atmosphere and changes to the climate system - and that this will increase in the future.

This is false. Research examining science on climate change established that 97% of papers published during 1991-2011 supported the positions that humans are causing climate change.

I agree with VictorVenema and think that this statement is misleading. There is overwhelming evidence that human-caused emissions have been the dominant cause of warming since the mid-20th century, as noted by the IPCC ( SPM 1.2 in The Synthesis Report Summary for Policymakers.

While it is true that the Earth has gone through periods of higher carbon and hotter and colder temperatures, humans - and certainly anything resembling modern human civilization - have not. The discussion of modern climate change and its impacts is primarily concerned with the survival of humans and modern flora and fauna. To evoke climate changes on a geologic timescale is misleading, especially when using "we" as though people were alive and surviving during these other periods (which is false).

Also misleading. Natural variability is still part of the ongoing climate warming. While the late 1970s were cooler than the years around 1904, it was not a continuous cooling, as seen below. You can also see in this figure that late 1970s temperatures were still high in comparison to the previous 120 years.

This image adds to the confusion on what Taylor means by "polar ice caps". It looks like there many be some snow covered sea ice in the foreground and then snow and ice covered land for most of the image. The article is really discussing sea ice, but this image gives the impression he is discussing land ice.

As noted, this is a meaningless comparison. The area coverage (extent) of sea ice can be large due to cold air temperatures, but form little sea ice volume. In other words, there's just a thin cover of sea ice across a large area. However, this thin seasonal ice can also melt quickly. This is why scientists look at the annual sea ice minimum extent and compare that over decades. Again, volume is also a better measure of sea ice quantity.

The focus on areal extent of ice - both sea ice and land ice - is also misleading. Both sea ice and land ice have thickness as well as area. This is why scientists very commonly measure volume changes. It turns out that Arctic sea ice and the Greenland and Antarctic Ice Sheets have all lost significant volume as sea ice thins and the coastal regions of ice sheets thin.

This is flat out false. NASA has an excellent visualization and graph of Arctic sea ice decline since 1979 here. Antarctic sea ice, on the other hand, has had some expansion. However, this expansion does not negate climate change, as explained in this news article.

As rgrumbine notes, Taylor uses the term "polar ice caps" to refer to both sea ice and land ice at different points during the article. This is a serious and misleading mistake as the two are very different. Scientists actually rarely use the term "polar ice caps", instead referring to sea ice (ice formed by freezing ocean water), ice sheets (Greenland and Antarctic Ice Sheets), or glaciers and ice caps (this would refer to land ice, such as the Juneau Icefield in Alaska or Devon Ice Cap in Canada). His incorrect use of "polar ice caps" is a significant problem.

As noted by lenae101, there is ample evidence and research to backup these statements regarding climate change. In addition to the IPCC reports, people can look to a wide variety of other reputable sources for information on the expected impacts of climate change. For example, the American Association for the Advancement of Science and The Royal Society. I suggest browsing the author and reference lists in these reports to get a sense of the truly global scientific agreement on the reality of climate change and it's significant influence on human life, economy, and environment.

Beyond Exit Glacier, we can also look at studies across the globe to confirm that glacier retreat - largely resulting from human caused warming - is occurring worldwide.

For example:

Howat, I. & Eddy, A Multi-decadal retreat of Greenland’s marine-terminating glaciers. Journal of Glaciology 57, 389 (2011).

Scambos, T. A. et al. Detailed ice loss pattern in the northern Antarctic Peninsula: widespread decline driven by ice front retreats. The Cryosphere 8, 2135–2145 (2014).

Marshall, S. Glacier retreat crosses a line. Science 345, 872–872 (2014). (And the associated paper it describes).

I absolutely agree with the other comment on this point. It is possible - and even appropriate and important - to care about all of these issues. This need not be a one or the other scenario. Given it's global reach, climate change will, however, impact people on many fronts - agriculture, water availability, heat waves, sea level rise - and requires immediate and global attention.

Both Alexis_b's and jmshea's comments on this point are excellent. Shrinking before widespread industrialization does not negate that recent ice loss is influenced by human caused climate warming.

This is absolutely false. We can look to the work of thousands of scientists around the globe using state of the art climate models and observations to find overwhelming evidence that we are in a warming trend and can expect warming to continue. These conclusions are based on well understood chemistry and physics. The IPCC reports provide a nice summary.