Frequently Asked Questions

Frequently Asked Question 5.1

Is Sea Level Rising?

Yes, there is strong evidence that global sea level gradually 

rose in the 20th century and is currently rising at an increased 

rate, after a period of little change between AD 0 and AD 1900. 

Sea level is projected to rise at an even greater rate in this century. 

The two major causes of global sea level rise are thermal expansion of the oceans (water expands as it warms) and the loss of 

land-based ice due to increased melting.

Global sea level rose by about 120 m during the several millennia that followed the end of the last ice age (approximately 

21,000 years ago), and stabilised between 3,000 and 2,000 years 

ago. Sea level indicators suggest that global sea level did not 

change significantly from then until the late 19th century. The 

instrumental record of modern sea level change shows evidence 

for onset of sea level rise during the 19th century. Estimates for 

the 20th century show that global average sea level rose at a rate 

of about 1.7 mm yr–1. 

Satellite observations available since the early 1990s provide 

more accurate sea level data with nearly global coverage. This 

decade-long satellite altimetry data set shows that since 1993, sea 

level has been rising at a rate of around 3 mm yr–1, significantly 

higher than the average during the previous half century. Coastal 

tide gauge measurements confirm this observation, and indicate 

that similar rates have occurred in some earlier decades.

In agreement with climate models, satellite data and hydrographic observations show that sea level is not rising uniformly 

around the world. In some regions, rates are up to several times the 

global mean rise, while in other regions sea level is falling. Substantial spatial variation in rates of sea level change is also inferred 

from hydrographic observations. Spatial variability of the rates of 

sea level rise is mostly due to non-uniform changes in temperature 

and salinity and related to changes in the ocean circulation. 

Near-global ocean temperature data sets made available in 

recent years allow a direct calculation of thermal expansion. It 

is believed that on average, over the period from 1961 to 2003, 

thermal expansion contributed about one-quarter of the observed 

sea level rise, while melting of land ice accounted for less than 

half. Thus, the full magnitude of the observed sea level rise during 

that period was not satisfactorily explained by those data sets, as 

reported in the IPCC Third Assessment Report.

During recent years (1993–2003), for which the observing 

system is much better, thermal expansion and melting of land 

ice each account for about half of the observed sea level rise, 

although there is some uncertainty in the estimates. 

The reasonable agreement in recent years between the observed 

rate of sea level rise and the sum of thermal expansion and loss of 

land ice suggests an upper limit for the magnitude of change in 

land-based water storage, which is relatively poorly known. Model results suggest no net trend in the storage of water over land 

due to climate-driven changes but there are large interannual and 

decadal fluctuations. However, for the recent period 1993 to 2003, 

the small discrepancy between observed sea level rise and the sum 

of known contributions might be due to unquantified humaninduced processes (e.g., groundwater extraction, impoundment in 

reservoirs, wetland drainage and deforestation). 

Global sea level is projected to rise during the 21st century at 

a greater rate than during 1961 to 2003. Under the IPCC Special 

Report on Emission Scenarios (SRES) A1B scenario by the mid2090s, for instance, global sea level reaches 0.22 to 0.44 m above 

1990 levels, and is rising at about 4 mm yr–1. As in the past, sea 

level change in the future will not be geographically uniform, 

with regional sea level change varying within about ±0.15 m of 

the mean in a typical model projection. Thermal expansion is projected to contribute more than half of the average rise, but land 

ice will lose mass increasingly rapidly as the century progresses. 

An important uncertainty relates to whether discharge of ice from 

the ice sheets will continue to increase as a consequence of accelerated ice flow, as has been observed in recent years. This would 

add to the amount of sea level rise, but quantitative projections of 

how much it would add cannot be made with confidence, owing 

to limited understanding of the relevant processes.

Figure 1 shows the evolution of global mean sea level in 

the past and as projected for the 21st century for the SRES A1B 

scenario. 

FAQ 5.1, Figure 1. Time series of global mean sea level (deviation from the 

1980-1999 mean) in the past and as projected for the future. For the period before 

1870, global measurements of sea level are not available. The grey shading shows 

the uncertainty in the estimated long-term rate of sea level change (Section 6.4.3). 

The red line is a reconstruction of global mean sea level from tide gauges (Section 

5.5.2.1), and the red shading denotes the range of variations from a smooth curve. 

The green line shows global mean sea level observed from satellite altimetry. The 

blue shading represents the range of model projections for the SRES A1B scenario 

for the 21st century, relative to the 1980 to 1999 mean, and has been calculated 

independently from the observations. Beyond 2100, the projections are increasingly 

dependent on the emissions scenario (see Chapter 10 for a discussion of sea level 

rise projections for other scenarios considered in this report). Over many centuries or 

millennia, sea level could rise by several metres (Section 10.7.4).
From the report accepted by Working Group I 

of the Intergovernmental Panel on Climate Change 

but not approved in detail

Frequently Asked Questions

FAQ Citation:

These Frequently Asked Questions have been taken directly from the chapters of the underlying report and are collected here. 

When referencing specific FAQs, please reference the corresponding chapter in the report from whence the FAQ originated.

When referencing the group of FAQs, please cite as:

IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment 

Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, 

M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.