A Look at the Camp Wildfire in Paradise

In early December I surveyed the horrific Camp Fire disaster in Paradise. Having been director for 25 years of a university field station located in the heart of the Tahoe National Forest, I’ve been a “student” of fire ecology for 30 years and wanted a closer look at why row after row of homes completely incinerated while surrounding trees were merely scorched, with leaves and needles browned but not burnt? 

Torched Homes in Camp Fire with Spared Trees

Large fires have recently ravaged about 1.8 million California acres a year, prompting media and politicians to proclaim a “new normal” that’s “evidence of global warming”.  But UC Berkeley fire ecologists have calculated that before 1800, fires burned 4 million California acres each year (despite cooler temperatures). So what natural fire dynamics promote such extensive burning?

Wildfires have indeed increased since 1970, but that’s relative to previous decades of intensive fire prevention. As fire was recognized as a natural and necessary phenomenon for healthy ecosystems a new era began. In the 70s the US Forest Service moved away from extinguishing all fires by 10 AM the day after detection, switching to a “let it burn policy” if human structures were not endangered.

Paradise, unfortunately, sprung up amidst a forest dominated by Ponderosa pines. Largely due to frequent lightning strikes and dry summers, Ponderosa habitat endures fires about every 11 years. Fortunately for California’s coastal residents, lightning is rare. However, both regions are vulnerable to human ignitions, which start 85-95% of all fires. Recognizing this growing problem, a bipartisan bill was presented to Governor Brown two years ago to secure our power grid. Shockingly he vetoed it. That was a bad choice given the Camp Fire, Wine Country Fires and many more were sparked by an ageing electrical infrastructure. Recent studies show larger fires result from a confluence of human ignitions and high winds. But it is not just random coincidence. The high winds that spread these massive fires also blow down power lines that ignite those fires.

In 2008 the world’s foremost expert on fire history, Stephen Pyne lamented, “global warming has furnished political cover to encourage certain fire management decisions while allowing climate to take the blame.” How true. Both PGE and Governor Brown have blamed wildfires on climate change. 

When you build a camp fire, you intuitively understand fire ecology basics. You do not hold a match to a log no matter how dry. You start a camp fire with kindling. Fire ecologists call forest kindling, like dead grass, leaves and small shrubs, “fine fuels”. In dry weather “fine fuels” become highly combustible in a matter of hours, or at most days, even during the winter. Furthermore, California’s summer climate is naturally dry for 3-4 months, creating highly combustible habitat each and every summer.

Additionally, camp fires only smolder without enough air, so we huff and puff to get a burst of flames. Likewise, high winds turn a spark into a major conflagration. It was strong winds that rapidly spread the Camp Fire. The fast-moving flames, feeding on “fine fuels” littering the forest floor, generated enough heat to ignite flammable homes that then burned from the inside out; but only enough heat to char the bark of most surrounding trees.

Miraculously spared buildings dotting a devastated landscape made the case for creating “defensible spaces” by managing the “fine fuels”. Surveying one unscathed church, the fire clearly came within 100 feet, scorching the base of every encircling tree. But due to a parking lot and a well-manicured lawn, the lack of “fine fuels” stopped the fire in its tracks. Trees on the lawn were not even charred. The public would benefit greatly if wildfire news stories emphasized the need to create adequate defensible spaces. 

With high deserts to the east and the ocean to the west, California’s winds shift with the seasons. Land temperatures always change faster than the ocean’s. In the summer, warmer land surfaces draw in moist sea breezes. The resulting fog moistens coastal landscapes and reduces fire danger there. Thus, any warming, whether natural or CO₂driven, should increase the fog.

In the autumn, the land cools faster than the ocean causing the winds to reverse direction. The colder it gets, the stronger the winds blow from the high deserts towards the coast, peaking in December. These winds are called Santa Annas in southern California. The Wine Country fires were spread by the Diablo winds. But regardless of the name, the science is the same.  Accordingly, it was November winds that fanned a spark into an inferno aimed directly at the heart of Paradise. 

It has long been known that due to these autumn and winter winds, much of California endures a dangerous fire season year-round.  On the optimistic side, any warming of the land during the cool seasons, whether natural or CO₂ driven, should reduce these winds. Indeed, the natural drivers of wildfire are very complex, and maintaining a defensible space is our safest bet.

Dr Judith Curry and Dr. Patrick Moore demolish Michael Mann in climate debate!

Here is the Transcript from Dr Judith Curry's statement from her debate along with Dr. Patrick Moore versus Michael Mann and Admiral Titley .   Well worth the read

reblogged from Dr. Curry's website https://judithcurry.com/2018/06/12/the-debate-mann-titley-moore-curry/

1    Cover

Good evening everyone.  Thank you very much for coming, I look forward to our conversation this evening.

2   Agreement/disagreement

There is widespread agreement on these basic tenets:

• Surface temperatures have increased since 1880
• Humans are adding carbon dioxide to the atmosphere
• Carbon dioxide and other greenhouse gases have a warming effect on the planet

However, there is substantial disagreement about the issues of greatest consequence:

• Whether the recent warming has been dominated by human causes
• How much the planet will warm in the 21stcentury
• Whether warming is ‘dangerous’
• How we should respond to the warming

I have bolded the two issues that are the focus of this conversation.

Now there is nothing wrong or bad about scientific disagreement.  In fact, the scientific process thrives in the face of disagreement, which motivates research in new directions.

3   Disagreement: causes of climate change

On the left hand side is the perspective of a stable climate that changes in response to changes in atmospheric CO2.  In other words, carbon dioxide as the climate control knob.  It’s a simple and seductive idea.

However some scientists think that this is a misleading oversimplification.  They regard climate as a complex nonlinear dynamical system, with no simple cause and effect.  Climate can shift naturally in unexpected ways, owing to natural internal variability associated with large-scale ocean circulations.

4    Elephant

Now these two perspectives are not mutually exclusive. Proponents of the CO2as control knob idea acknowledge the existence natural variability but dismiss it as noise that averages out.  Proponents of the natural variability arguments acknowledge the impact of CO2, but consider it to be a modest wedge that projects onto the natural modes of climate variability.

The point of this cartoon is that if you only look at one part of the elephant, you will misdiagnose.  You need to look at the entire elephant.

The bottom line is that we don’t yet have a unified theory of climate variability and change that integrates all this.

5    Disagreement: cause of climate change

So does this rather arcane scientific debate actually matter?  Well, yes it does.

If you assume that carbon dioxide is the control knob for climate, than you can control climate by reducing CO2emissions.

If you assume that climate change primarily occurs naturally, then the Earth’s climate is largely uncontrollable, and reducing CO2emissions will do little or nothing to change the climate.

My personal assessment aligns with the right-hand side, emphasizing natural variability.  However, the IPCC and the so-called consensus aligns with the left hand side.  About 10 years ago, I also aligned with left hand side, because I thought supporting the IPCC consensus was the responsible thing to do.

Here is how and why I changed my mind.

6    Policy cart before scientific horse

In 2010, I started digging deeper, both into the science itself and the politics that were shaping the science.  I came to realize that the policy cart was way out in front of the scientific horse.

The 1992 UN Climate Change treaty was signed by 190 countries before the balance of scientific evidence suggested even a discernible human influence on global climate.  The 1997 Kyoto Protocol was implemented before we had any confidence that most of the warming was caused by humans.  There was tremendous political pressure on the IPCC scientists to present findings that would support these treaties, which resulted in a manufactured consensus.

7     You find what you shine a light on

Here is how the so-called consensus and increasing confidence in human-caused global warming became a self-fulfilling prophesy.

You find what you shine a light on.  In other words, we have only been looking at one part of the elephant.

Motivated by the UN Climate treaty and the IPCC and government funding, climate scientists have focused primarily on human-caused climate change.  Other factors important for understanding climate variability and change have been relatively neglected. I have highlighted long-term ocean oscillations and solar indirect effects, since I think that these are potentially very important on decadal to century timescales.

8     The sea level rise alarm

One of the most consequential impacts of a warming climate is sea level rise. These two statements by climate scientists typify the alarm over sea level rise:

Is this alarm justified by the scientific evidence?

9 Is CO2 the control knob for global sea level rise?

This figure illustrates the challenge of attributing long-term sea level rise to CO2emissions. The blue curve shows sea level change since 1800, measured from tide gauges.

The red curve shows global emissions of carbon dioxide from burning of fossil fuels. You can see that global sea levels were rising steadily long before fossil fuels emissions became substantial. You can also see that the steep increase in emissions following 1950 is associated with very little sea level rise between 1950 and 1990.

An uptick in sea level rise occurred in the 1990’s, which is circled.  Lets take a closer look to see what is causing this.

10   What is causing recent sea level rise?

Since 1993, global satellite data have provided valuable information about sea level variations and glacier mass balance.  This figure shows a recent analysis of the budget of sea level rise since 1993.  You can see that overall the rate of sea level rise has increased since 1993.

What is causing this increase?  The turquoise region on the bottom of the diagram relates directly to expansion from warming.  You actually see a decrease until about 2009, which has been attributed to the cooling impact following the eruption of Mount Pinatubo in 1992.

What stands out as causing the increase in the rate of sea level rise is the growing contribution from Greenland, which is the dark blue area on top.  Hence the recent increase in the rate of sea level rise is caused by Greenland melting.

11  Variations in Greenland glacier mass balance

So, is the Greenland melting caused by increasing CO2 emissions?

This figure shows the Greenland mass balance for the 20th century. Ice sheet mass balance is defined as increase from snowfall, minus the decrease from melting.  You can see the negative mass balance values after 1995, reflecting mass loss that raises sea level.  If you look earlier in the record, you see even larger negative values particularly in the 1920’s and 1930’s.  Clearly, the high surface mass loss rates of recent years are not unprecedented, even in the 20thcentury.

Greenland was anomalously warm in the 1930’s and 1940’s. What caused this?

The bottom figure shows variations in the Atlantic Multidecadal Oscillation, which is an important mode of natural internal climate variability.  The AMO is a powerful control on the climate of Greenland.

Ingeneral, years with positive AMO index are associated with a mass loss for Greenland, whereas negative AMO index is associated with a mass gain.

12  IPCC AR5 quotes on sea level rise

From this analysis, I can only conclude that CO2 emissions are not the main cause of sea level rise since the mid 19thcentury.

The scientific evidence that I’ve shown you on the preceding slides is well known to the IPCC.  Here are some statements that the most recent IPCC report made on sea level change and Greenland: 

13 To what extent are man-made CO2 emissions contributing to climate change?

I’ve been asked to respond to the question “To what extent are man-made CO2 emissions contributing to climate change?”

The short answer is:  ‘we don’t know.’ The reason is that we don’t know how to disentangle natural internal variability from the effects of CO2–driven warming

Even the IPCC doesn’t claim to know exactly. The most recent IPCC assessment report says it is ‘extremely likely’ to be  ‘more than half.’ ‘More than half’ is not very precise.

Given the IPCC’s neglect of multi-decadal and longer time scales of natural internal variability, I regard the extreme confidence of their conclusion to be unjustified

So here is my personal assessment, using the jargon of the IPCC:  Man-made CO2emissions are as likely as not to contribute less than 50% of the recent warming

14  Should we reduce emissions to prevent warming?

Even if you believe the climate model projections, there is still genuine disagreement regarding whether a rapid acceleration away from fossil fuels is the appropriate policy response.

One side argues that reducing CO2emissions are critical for preventing future dangerous warming of the climate.  The other side argues that any reduction in warming would be minimal and at high cost, and that the  ‘cure’ could be worse than the ‘disease’.

15   Climate pragmatism

What makes most sense to me is Climate Pragmatism, which has been formulated by the Hartwell group.  Climate pragmatism has 3 pillars:

• Accelerate energy innovation
• Build resilience to extreme weather
• No regrets pollution reduction

These policies provide near-term socioeconomic & environmental benefits and have justifications independent of climate mitigation & adaptation

 These are no regrets policies that do not require agreement about climate science or the risks of uncontrolled greenhouse gases

16   Madhouse effect

I would like to make a few comments on the state of the scientific and public debate on climate change.

Here is my take on the Madhouse effect.  The madhouse that concerns me is one that has been created by climate scientists.  The madhouse is characterized by

• Rampant overconfidence in an overly simplistic theory of climate change
• Enforcement of a politically-motivated, manufactured ‘consensus’
• Attempts to stifle scientific and policy debates
• Activism and advocacy for their preferred politics and policy
• Self-promotion and ‘cashing in’
• Public attacks on other scientists that do not support the ‘consensus’

Hmmm . . . maybe I should write a book.

Will Advances in Groundwater Science Force a Paradigm Shift in Sea Level Rise Attribution

In a 2002 paper, what is frequently referred to as “Munk’s enigma”, Scripps Institution of Oceanography’s senior researcher bemoaned the fact researchers could not fully account for the causes of sea level rise. He lamented, “the historic rise started too early, has too linear a trend, and is too large.” Early IPCC analyses noted about 25% of estimated sea level rise was unaccounted for. Accordingly, in 2012, an international team of prominent sea level researchers published, Twentieth-Century Global-Mean Sea Level Rise: Is the Whole Greater than the Sum of the Parts? (henceforth Gregory 2012). They hoped to balance struggling sea level budgets by re-analyzing and adjusting estimates of the contributions from melting glaciers and ice caps, thermal expansion, and the effects of dam building and groundwater extraction. However, a natural contribution from any imbalance in groundwater re-charge vs discharge was never considered. Yet the volume of freshwater stored as groundwater, is second only to Antarctica’s frozen supply, and 3 to 8 times greater than Greenland’s.

At the risk of oversimplifying, the effects of groundwater storage can be differentiated between shallow-aquifer effects that modulate global sea level on year to year and decade to decade timeframes, versus deep aquifer effects that modulate sea level trends over centuries and millennia.

Researchers are increasingly aware of natural shallow groundwater dynamics. As noted by Reager (2016) in A Decade of Sea Level Rise Slowed by Climate-Driven Hydrology, researchers had determined the seasonal delay in the return of precipitation to the oceans causes sea levels to oscillate by 17 ± 4 mm [~0.7 inches] per year. Reager (2016) also argued decadal increases in terrestrial water storage driven by climate events such as La Nina, had reduced sea level rise by 0.71 mm/year. Likewise, Cazenave 2014 had published according to altimetry data, sea level had decelerated from 3.5 mm/yr in the 1990s to 2.5mm/yr during 2003-2011, and that deceleration could be explained by increased terrestrial water storage, and the pause in ocean warming reported by Argo data.

Improved observational data suggest during more frequent La Nina years a greater proportion of precipitation falls on the land globally and when routed through more slowly discharging aquifers, sea level rise decelerates. During periods of more frequent El Niños, more rain falls back onto the oceans, and sea level rise accelerates. In contrast to La Nina induced shallow-aquifer effects, deep aquifers have been filled with meltwater from the last Ice Age, and that water is slowly and steadily seeping back into the oceans today.

Munk’s “Too Linear Trend” Enigma and Deep Groundwater Discharge

Hydrologists concerned with sustainable groundwater supplies and drinking water contamination, have been in the forefront of analyzing the volume and ages of the world’s groundwater, providing greater insight into deep aquifer effects.  Gleeson (2015) determined, “total groundwater volume in the upper 2 km of continental crust is approximately 22.6 million cubic kilometers, twice as much as earlier estimates. If all 22.6 million cubic kilometers of freshwater stored underground reached the oceans, sea level would rise 204 feet (62,430 millimeters). Via various isotope analyses and flow models, Jasechko (2017) estimated that between 42-85% of all groundwater stored in the upper 1 kilometer of the earth’s crust is water that had infiltrated the ground more than 11,000 years ago, during last Ice Age.

Clearly the earth’s groundwater has yet to reach an equilibrium with modern sea levels. With deep aquifer discharge primarily regulated by geological pore spaces (in addition to pressure heads), the slow and steady discharge of these older waters affects sea level rise on century and millennial timeframes. And, although freshwater discharge from deep aquifers may be locally insignificant relative to river runoff, deep aquifer discharge when integrated across the globe could account for the missing contribution to the sea level rise budgets.

Unfortunately quantifying the groundwater discharge contribution to sea level rise is extremely difficult, suffering from a low signal to noise problem. That difficulty is why natural groundwater contributions are often ignored or brushed aside as insignificant. Although GRACE satellite monitoring of gravity changes offers great promise for detecting changes in terrestrial groundwater storage, GRACE cannot accurately separate the relatively small discharge of deep aquifers from large annual changes in shallow groundwater. In periods of heavy rains, groundwater increases will mask deep aquifer discharge. And during a drought, any deep groundwater discharge will likely be attributed to the lack of rain.

However, estimates of groundwater re-charge via isotope analyses can provide critical information regards rates of groundwater re-charge and discharge.

Using the abnormal levels of tritium released during nuclear testing in the 1950s, plus carbon­14 dating, researchers have categorized the time since groundwater had last left the surface into 25, 50, 75 and 100-year old age classes. As expected, the youngest water is concentrated in the shallowest aquifer layers and the proportion of young water decreases with depth. The estimated volume of 25-year-old or younger groundwater suggests global groundwater is currently recharging at a rate that would reduce sea level by 21 mm/year (0.8 inches/year). Water cycle researchers  (i.e. Dai and Trenberth) have made the dubious assumption that the amount of water transported via precipitation to the land from the ocean is balanced each year by river runoff.  But if the tritium derived estimates are valid, balancing water cycle and sea level budgets becomes more enigmatic. Clearly a significant amount of precipitation does not return for decades and centuries.

Intriguingly, comparing the smaller volume of ground water aged 50 to 100-years-old versus the volume of water 50-years-old and younger suggests 2 possible scenarios. Either ground water recharge has increased in recent decades, or if recharge rates averaged over 50 years have remained steady, then as groundwater ages a significant portion seeps back to the ocean at rates approaching 1.7 mm/year, a rate that is very similar to 20^th century IPCC estimates of sea level rise.

Groundwater discharge must balance recharge or else it directly alters global sea levels. When less than 21 mm/year seeps back to the ocean, then natural groundwater storage lowers sea level. When discharge is greater than 21 mm/year, then groundwater discharge is raising sea level. Without accounting for recharge vs discharge, the much smaller estimates of all the other factors contributing to sea level rise are simply not well constrained.

Higher rates of discharge could account for the enigmatic missing sea level contributions reported by the IPCC and other researchers (i.e. Gregory 2012). More problematic, if discharge proves to significantly exceed recharge, then estimates of contributions from other sources such as melting ice and thermal expansion may be too high. What is certain, the current estimates of contributions to sea level from melting ice and thermal expansion only range from 1.5 to 2.0 mm/year, and those factors by themselves cannot offset the tritium estimated 21 mm/year of groundwater recharge. So, what is missing in our current water cycle budgets?

The Importance of Submarine Groundwater Discharge (SGD)

The recharge-discharge imbalance can be reconciled if water cycle budgets included the difficult-to-measure rates of prolific submarine groundwater discharge (SGD). Freshwater springs bubbling up from coastal sea floors have long been observed. To reliably replenish drinking water, Roman fisherman mapped their occurrences throughout the Mediterranean. Moosdorf (2017) has reviewed the locations and many human uses of fresh submarine groundwater discharge around the world.

Recent ecological studies have measured local submarine groundwater seepages to determine contributions of solutes and nutrients to coastal ecosystems. But those sparse SGD measurements cannot yet be reliably integrated into a global estimate. Rodell (2015) notes that most water cycle budgets have ignored SGD due to its uncertainty, so Rodell’s water cycle budget included a rate of SGD equivalent to 6.5 millimeters/year (~0.25 inch/yr) of sea level rise. However, that estimate is still insufficient to balance current recharge estimates.

However, with improving techniques, researchers recently estimated total submarine groundwater (saline and fresh water combined) discharges suggesting a rate 3 to 4 times greater than the observed global river runoff, or a volume equivalent to 331 mm/year (13 inches) of sea level rise. Nonetheless more than 90% of that submarine discharge is saline sea water, most of which is likely recirculated sea water, and not likely to affect sea level. Only the fraction of entrained freshwater would raise sea level. To balance the 21 mm/year ground water recharge, between 6 and 7% of total SGD must be freshwater and that amount is very likely. Local estimates of the freshwater fraction of submarine discharge range from 1 to 35%, and on average just less than 10%. If fresh submarine groundwater discharge approaches just 7% of the total SGD, it would not only balance current groundwater recharge, but would steadily raise sea level by an additional 2 mm/year, even if there was no ocean warming and no melting glaciers.

A Sea Level Rise “Base-flow” and Paleo-climate Conundrums

Hydrologists seek to quantify the aquifer contributions to river flow, otherwise known as the “base flow”. During the rainy season or the season of melting snow, any groundwater contribution is masked by heavy surface runoff and shallow aquifer effects. However, during extended periods of drought hydrologists assume the low river flow that persists must be largely attributed to supplies from deeper aquifers. Streams that dry up during a drought are usually supported by small shallow aquifers, while reduced but persistent river and stream flows must be maintained by large aquifers. Using a similar conceptual approach, we can estimate a possible “base flow” contribution to sea level.

When the continental ice sheets began to melt as the earth transitioned from its Ice Age maximum to our present warm interglacial, sea level began to rise from depths ~130 meters lower than today (see graph below). Melting continental ice sheets drove much higher rates of sea level rise than seen today, ranging from 10 to 40+ mm/year. Approximately 6,000 years ago, a consensus suggests the last of the continental ice sheets had melted completely, the earth’s montane glaciers had disappeared, and Greenland and Antarctic ice sheets had shrunk to their minimums. The earth then entered a long-term 5000-year cooling trend dubbed the Neoglaciation. Although sea level models forced only by growing glaciers and cooling ocean temperatures would project falling sea levels, proxy evidence enigmatically suggests global sea level continued to rise. Albeit at reduced rates, global sea level continued to rise another 4 meters (Figure 1 below). Although there is some debate regards any continued contribution from Antarctica and “ocean siphoning”, according to Lambeck 2014 about 3 meters of sea level were added between 6.7–4.2 thousand years ago. That continued sea level rise could be explained by aquifer discharge, suggesting a minimal “base flow” of ~1.2 mm/year from groundwater discharge.

Similarly, during the Little Ice Age between 1300 and 1850 AD, montane glaciers as well as Greenland and Antarctic ice sheets, grew and reached their largest extent in the last 7,000 years. Ocean temperatures cooled by about 1 degree. Yet inexplicably, most researchers estimate global sea level never dropped significantly. They report sea levels were “stable” during the Little Ice Age, fluctuating only by tenths of a millimeter. That stability contrasts greatly with the recent rising trend, that has led some to attribute the current rise to increasing CO2 concentrations. However Little Ice Age stability defies the physics of cooling temperatures and increasing water storage in growing glaciers that should have caused a significant sea level fall. However, that seeming paradox is consistent with a scenario in which a “base flow” from groundwater discharge would offset any transfer of waters to growing Little Ice Age glaciers.

Once the growth of Little Ice Age glaciers stopped, and groundwater base flow was no longer offset, we would expect sea levels to rise as witnessed during the 19^th and 20^th centuries. Such a scenario would also explain Munk’s enigma that sea level rise had started too early, before temperatures had risen significantly from any CO2-driven warming.

Interestingly, assuming a ballpark figure of a 1.2 mm/year groundwater base flow, unbalanced groundwater discharge could also explain the much higher sea levels estimated for the previous warm interglacial, the Eemian. Researchers estimate sea levels ~115,000 years ago were about 6 to 9 meters higher than today. That interglacial has also been estimated to have spanned 15,000 years before continental glaciation resumed. Compared to our present interglacial span of 11,700 years, an extra 3,300 years of groundwater discharge before being offset by resumed glacier growth, could account for 4 meters of the Eemian’s higher sea level.

Recent glacier meltwater contribution to sea level is likely overestimated?

In addition to a groundwater base flow driving the current steady rise in sea level, meltwater from retreating Little Ice Age glaciers undoubtedly contributed as well. But by how much? Researchers have estimated there was greater glacial retreat (and thus a greater flux of meltwater) in the early 1900s compared to now. So, current glacier retreat is unlikely to cause any acceleration of recent sea level rise. Furthermore, we cannot assume glacier meltwater rapidly enters the oceans. A large proportion of meltwater likely enters the ground, so it may take several hundred years for Little Ice Age glacier meltwater to affect sea level.

How fast can groundwater reach the ocean? Groundwater measured in the Great Plains’ Ogallala Aquifer can flow at a higher-than-average seepage rate of ~300 mm (~1 foot) in a day, or about the length of a football field in a year.  For such “fast” moving groundwater to travel 1000 kilometers (620 miles) to the sea, it would require over 10,000 years! Most ground water travels much slower. The great weight of the continental glaciers during our last ice age, applied such great pressure that it forced meltwater to into the ground at much greater rates than currently observed recharge. And that Ice Age meltwater is still slowly moving through aquifers like the Ogallala.

(However, its release to the ocean has been sped up by human pumping. Recent estimates suggest that globally, human groundwater extraction currently exceeds rates of water capture from dam building, so that groundwater depletion is now accelerating sea level rise.)

How much of the current meltwater can we expect to transit to the ocean via a slow groundwater route? That’s a tough question to answer. However, thirteen percent of the earth’s ice-free land surface is covered by endorheic basins as illustrated by the gray areas shown in the illustration below. Endorheic basins have no direct outlets to the ocean. Water entering endorheic basins only return to the sea via evaporation, or by the extremely slow route of groundwater discharge. Any precipitation or glacial meltwater flowing into an endorheic basin could require centuries to thousands of years to flow back to the oceans.

For example, in 2010-2011, researchers reported that a La Nina event had caused global sea level to fall by the equivalent of 7mm/year (~0.3 inches/year). That dramatic drop happened despite concurrent extensive ice melt in Greenland and despite any base flow contribution.  As described by Fasullo (2013), GRACE satellite observations detected increased groundwater storage caused by higher rates of rainwater falling on endorheic basins, primarily in Australia. Although satellite observations suggested much of the rainwater remained in the Australian basin, sea level resumed its unabated rise as groundwater base flow contribution would predict.

To balance their sea level budgets, researchers assert melting glaciers have added ~0.8 mm/year to recent sea level rise. The 20^th century retreat of most glaciers is undeniable, but we cannot simply assume all 20^th century glacier meltwater immediately reached the oceans. The greatest concentration of ice, outside of Greenland and Antarctica, resides in the regions north of India and Pakistan, in the Himalaya and Karakoram glaciers. Most melt water flowing northward enters the extensive Asian endorheic basins. Likewise, some of the Sierra Nevada meltwater flows into Nevada’s Great Basin, and some Andes meltwater flows into the endorheic basins of the Altiplano and Lake Titicaca as well as the Atacama Desert. It is very likely much of the current glacial meltwater will then take decades to millennia to reach the ocean and has yet to impact modern sea levels. If the glacial melt water contribution to sea level is overestimated, then, the unaccounted-for contribution to sea level rise becomes much larger than initially thought.

Accurate Attribution of Groundwater Discharge and Recharge Will Constrain Sea Level Contributions

Using a combination of GRACE gravity data that measured changes in ocean mass, altimetry data that measured changes in ocean volume and Argo data that measured heat content, Cazenave (2008) used 2 different methods and both estimated the contribution from increased ocean heat to be about 0.3 to 0.37 mm/year. Jevrejeva (2008) calculated a similar heat contribution. Other researchers suggest thermal expansion contributes 1.2 to 1.5 mm/year (i.e. Chambers 2016). Such large discrepancies reveal contributing factors to sea level rise are not yet reliably constrained.

One of the great uncertainties in sea level research are glacial isostatic adjustments.

Researchers have subjectively adopted various Glacio-isotatic adjustment models with recommended adjustments ranging from 1 to 2 mm/year. For example, although GRACE gravity estimates had not detected any added water mass to the oceans, Cazenave (2008) added a 2 mm/year adjustment, as illustrated from her Figure 1 below. Other researchers only added a 1 mm/yr adjustment.

In the Gregory (2012) paper Twentieth-Century Global-Mean Sea Level Rise: Is the Whole Greater than the Sum of the Parts? researchers suggested the sea level budget could be balanced and the IPCC’s unaccounted for contribution to sea level rise could be explained by making 5 assumptions:

1)    Assume the contribution from glacier melting was greater than previously estimated.

But greater melting rates were documented for the 30s and 40s, and the likelihood that some glacier meltwater is still trapped as groundwater, suggests the glacier meltwater contribution has been overestimated.

2)    Assume an increased contribution from thermal expansion.

But Argo data suggests the contribution from thermal expansion has been decreasing and plateauing.

3)    Assume Greenland positively contributed to sea level throughout the entire 20^th century.

Greenland has undoubtedly contributed to episodes of accelerating and decelerating sea level changes, but the greatest rate of Greenland warming occurred during the 1920s and 30s. Previous researchers suggested Greenland glaciers have oscillated during the 20^th century but had been stable from the 60s to 1990s.  Although there was increased surface melt in the 21^st century, culminating in 2012, that melt rate has since declined. And according to the Danish Meteorological Institute, Greenland gained about 50 billion tons of ice in 2017 which should have lowered sea level in 2017.  Clearly Greenland cannot explain the enigmatic steady 20th century sea level rise.

4)    Assume reservoir water storage balanced groundwater extraction.

But net contributions from groundwater extraction vs water impoundments and other landscape changes are still being debated. For the period 2002–2014 landscape changes have been estimated to have reduced sea level by −0.40 mm/year versus IPCC estimates of contributing 0.38 mm/year from 1993–2010 to sea level rise.

5)    Assume the remaining unaccounted contribution to sea level rise is small enough to be attributed to melting in Antarctica. 

Debatably, Antarctic melting is too often used as the catch-all fudge factor to explain the unexplainable. Furthermore, there is no consensus within the Antarctic research community if there have been any human effects on Antarctica’s ice balance. Regions that are losing ice are balanced by regions that are gaining ice. Claims of net ice loss have been countered by claims of net ice gain such as NASA 2015. Additionally, unadjusted GRACE gravity data has suggested no lost ice mass and all estimates of ice gains or loss depend on which Glacial Isostatic Adjustments modelers choose to use. We cannot dismiss the possibility that unaccounted for groundwater discharge has been mistakenly attributed to hypothetical Antarctic melting? 

A better accounting of natural groundwater discharge is needed to constrain the range of contributions to sea level rise suggested by researchers such as Gregory 2012. The greater the contribution from groundwater discharge, the smaller the adjustments used to amplify contributions from meltwater and thermal expansion. Until a more complete accounting is determined, we can only appreciate Munk’s earnest concern. How can we predict future sea level rise if we don’t fully understand the present or the past?