As required by Section 106 of the U. S. Global Change Research Act of 1990, the U. S. Global Change Research Program has published the 2017 Climate Science Special Report, which is part of the Fourth National Climate Assessment.
The report is a full assessment of the science of climate change, with a focus on the United States. The report includes an executive summary written for a lay person audience along with 15 chapters of more technical information relating to global changes; physical drivers of climate change; detection and attribution of climate change; climate models; circulation and variability; temperature changes; precipitation changes; droughts, flood, and wildfires; extreme storms; land cover changes; arctic changes; sea level rise; ocean acidification; mitigation; and potential surprises. The report also includes large appendices with information on data; model weighting; detection and attribution methods; acronyms; and a glossary.
This post provides a summary of the technical information relating to precipitation, extreme storms, floods, and hurricanes. The post will use the vocabulary from the report used to describe the confidence we have around the predictions and trends discussed as well as the likelihood of predictions and trends occurring in the future the way we believe. The vocabulary is defined in the image below:
The report looks at precipitation in two ways. First, the report describes historical changes observed, based on gauge measurements. Second, the report describes possible future trends based on modeling results. Key findings are copied directly from the report below:
- Annual precipitation has decreased in much of the West, Southwest, and Southeast and increased in most of the Northern and Southern Plains, Midwest, and Northeast. A national average increase of 4% in annual precipitation since 1901 is mostly a result of large increases in the fall season. (Medium
- Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence). There are important regional differences in trends, with the largest increases occurring in the northeastern United States (high confidence). In particular, mesoscale convective systems (organized clusters of thunderstorms)—the main mechanism for warm season precipitation in the central part of the United States—have increased in occurrence and precipitation amounts since 1979 (medium confidence).
- The frequency and intensity of heavy precipitation events are projected to continue to increase over the 21st century (high confidence). Mesoscale convective systems in the central United States are expected to continue to increase in number and intensity in the future (medium confidence). There are, however, important regional and seasonal differences in projected changes in total precipitation: the northern United States, including Alaska, is projected to receive more precipitation in the winter and spring, and parts of the southwestern United States are projected to receive less precipitation in the winter and spring (medium confidence).
Historical changes vary by region and by season. The graphic below illustrates the observed changes in precipitation by calculating the difference between the average depth of rain during the period from 1986 to 2015, minus the average depth of rain during the period from 1901 to 1960, divided by the average for the 1901-1960 period.
The current average annual rainfall in the Houston area is reported as about 50 inches per year, but this is based on averaging the entire period of record. The maps above illustrate how the average changed from the 1901-1960 period to the 1986-2015 period. The scale of the map makes it a bit difficult to see the effect in the Houston region, but here’s what I see for the Houston area:
- Annual Precipitation: Historic increase of 5 to 10%, with perhaps an area to the east with historic increases of between 10 and 15%.
- Winter Precipitation: Historic increase of -5 to +5%.
- Spring Precipitation: Historic increase of 0 to 10%.
- Summer Precipitation: Historic increase of >15%.
- Fall Precipitation: Historic increase of >15%.
Just a quick reminder, these annual averages are not used in the design of our drainage systems or our floodplain management infrastructure. We use the so-called 1% annual chance 24-hour duration event, which is about 13 inches of rain in one day.
The frequency of extreme storms observed in the United States have either increased or decreased depending upon the region; in Texas, they have increased.
Observed Trends in 20-Year, 1-Day Events
The authors present information about the change in depth of 5% annual chance, 24-hour duration rain event (about 6.2 inches in Houston). The figure below shows historic changes in the 20-year return value (5% annual chance event) of the seasonal daily precipitation totals for the contiguous United States over the period 1948 to 2015 using data from the Global Historical Climatology Network.
The figure illustrates that the depth of rain associated with the 5% annual chance (20-year recurrence interval), 24-hour duration event in Texas has historically increased by 0.19 to 0.48 inches from 1948 to 2015 (depending upon the season). The current 5% annual chance event in Houston is 6.2 inches of rain in 24 hours, so these depth increases represent historic increases of about 3.0% to 7.7%.
Observed Trends in 20-Year, 2-Day Duration Events
What about the 5% annual chance (20-year recurrence interval), 2-day duration event? The figure below, which includes rain event observations from the entire contiguous United States – not just Texas, requires some explanation. It was created from a series of steps.
First, the authors looked up the number of 2-day duration rain events observed at rain gauges around the contiguous United States that exceeded the 5% annual chance, 2-day event depth at each of the associated gauges. Then they compared that number to the mean number of events larger than the 5% annual chance, 2-day event depth measured at each gauge during the period from 1901 to 1960. This is called the “Relative Number of Extreme Events” in the graph. Gauges experiencing more frequent 5% annual chance, 2-day rains during the period from 1960 on, would have a positive number and those experiencing less would have a negative number.
To see how these historic increases or decreases in the frequency of these storms vary with time, the authors grouped the data into five year periods called “pentads.” Each pentad is identified in the graph by a year that matches the end of each of the five year periods used in the calculation. These years are shown along the horizontal axis.
The authors also grouped the rain depth measurements at the various gauges spread across the United States into area grids and then into a single area for the entire United States. So this graphs does not tell us anything about historic regional differences.
Its pretty clear from the graph, that, on the whole, the United States experienced a higher number of 5% annual chance, 2-day duration, rain events in the later part of the 1900’s and early 2000’s than in the earlier part of the 1900’s.
Many factors influence projected precipitation amounts and patterns. The authors indicate that “projecting regional changes is much more difficult [than global changes] because of uncertainty in projecting changes in the large-scale circulation that plays an important role in the formation of clouds and precipitation.” For the contiguous United States, precipitation amounts will be a mix of increases, decreases, or little change, depending on location and season.
Globally, high-latitude regions are generally projected to become wetter while the subtropical zone is projected to become drier. Because the United States is located between these two regions, the authors state that “there is significant uncertainty about the sign and magnitude of future anthropogenic changes to seasonal precipitation in much of the region, particularly in the middle latitudes of the Nation.” They add that “confidence is high that precipitation extremes will increase in frequency and intensity in the future throughout the contiguous United States.”
The figure below illustrates the projected change (%) in seasonal precipitation as compared to the average precipitation measured during the period from 1976 to 2005. Stippling (dots) indicates that projected changes will be large compared to
natural variations. Hatching (diagonal lines) indicates that projected changes will be small compared to natural variations.
This graphic shows that Texas (and the Houston region) are expected to experience small changes in season precipitation as compared to natural variations (except perhaps for some portions of the Houston area that might experience 10 to 20% less rainfall in the spring of each year). All variations for the Houston area appear to range between -10 to +10%, except for the spring season.
The report presents the following key finding:
Detectable changes in some classes of flood frequency have occurred in parts of the United States and are a mix of increases and decreases. Extreme precipitation, one of the controlling factors in flood statistics, is observed to have generally increased and is projected to continue to do so across the United States in a warming atmosphere. However, formal attribution approaches have not established a significant connection of increased riverine flooding to human-induced climate change, and the timing of any emergence of a future detectable anthropogenic change in flooding is unclear. (Medium confidence).
Trends in measured peak stream flowrates vary across the contiguous United States. Data from 200 stream gauges indicates areas of both increasing and decreasing frequency of flooding but these data do not suggest that these trends are attributable to human influences. Significant increases in flooding frequency have been detected in about one-third locations in the central United States. Less significant increases in flooding magnitude were observed in the same gauge locations.
The authors explain the discrepancy between observed extreme precipitation increases and flooding trends by highlighting the seasonality of the two phenomena. “Extreme precipitation events in the eastern half of the United States are larger in the summer and fall when soil moisture and seasonal streamflow levels are low and less favorable for flooding. By contrast, high streamflow events are often larger in the spring and winter when soil moisture is high and snowmelt and frozen ground can enhance runoff.”
The authors add that “floods may be poorly explained by daily precipitation characteristics alone; the relevant mechanisms are more complex, involving processes that are seasonally and geographically variable, including the seasonal cycles of soil moisture content and snowfall/snowmelt.”
Regarding hurricanes (called “tropical cyclones” in the report), the authors state:
“It is likely that global mean tropical cyclone maximum wind speeds and precipitation rates will increase; and it is more likely than not that the global frequency of occurrence of tropical cyclones will either decrease or remain essentially the same.
Confidence in projected global increases of intensity and tropical cyclone precipitation
rates is medium and high, respectively, as there is better model consensus. Confidence is further heightened, particularly for projected increases in precipitation rates, by a robust physical understanding of the processes that lead to these increases.
Confidence in projected increases in the frequency of very intense tropical cyclones is generally lower (medium in the eastern North Pacific and low in the western North Pacific and Atlantic) due to comparatively fewer studies available and due to the competing influences of projected reductions in overall storm frequency and increased mean intensity on the frequency of the most intense storms.
Both the magnitude and sign of projected changes in individual ocean basins appears to depend on the large-scale pattern of changes to atmospheric circulation and ocean surface temperature. Projections of these regional patterns of change—apparently critical for tropical cyclone projections—are uncertain, leading to uncertainty in regional tropical cyclone projections.”