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The security risks posed by an alternative 'non-continuous' nuclear posture, as outlined in last week's Trident Alternatives Review, are inherently subjective. Financial (rather than strategic) arguments may come to dominate the alternatives debate - something the Liberal Democrats should bear in mind.
Last Tuesday, the Coalition government announced the results of its review into alternatives to a like-for-like replacement of the UK's 'Trident' nuclear system - a force of four nuclear-armed submarines maintaining a continuous patrol. This system is due to be replaced in the mid-2020s, with an expected price tag of between £17-23 billion (FY2013 prices), and an overall lifetime cost that could approach £97 billion.
Bearing this cost in mind, the review explores a number of alternative mechanisms and methods for operating a nuclear force that can provide a credible deterrent threat to potential aggressors. Overall, the review concludes that there are alternatives to the current system that can inflict such damage, ensuring that 'most potential adversaries around the world would be deterred'.
For the Liberal Democrat party, this is certainly a welcome conclusion. The party rejected a 'like-for-like' replacement to the Trident system in their manifesto for the 2010 election, and in a speech at RUSI last Tuesday, Danny Alexander - Liberal Democrat 'overseer' of the review - used the occasion of the review's launch to establish the foundations of the Liberal Democrat argument for an alternative approach to the UK's nuclear future.
Mr Alexander acknowledged that moving away from a system of nuclear submarines armed with ballistic missiles is not cost-effective at this time, and suggested that the UK should instead seek to end permanent continuous submarine patrols, allowing the procurement of fewer replacement submarines. In their place, the Liberal Democrats point towards four complementary postures outlined in the review that would vary the frequency and duration of patrols (from near-continuous to no regular patrols), and the ease in which the UK can move from one to another as necessary, depending on perceptions of the threats facing the UK at the time.
Whether the UK adopts this flexible approach to non-continuous patrolling will ultimately depend upon a parliamentary vote in 2016, when a final 'main gate' decision on the future of the UK's nuclear force will have to be made. The review alone cannot make this decision. As a study commissioned by two parties with 'very different approaches' to this issue, it avoids making any overt cost-benefit calculations about each alternative, and opens by declaring that it is not a statement of government policy. Rather, it leaves to 'political confidence' whether the UK should operate a non-continuous nuclear posture.
Non-Continuous Nuclear Postures
Central to questions of 'political confidence' in a non-continuous posture is the fact that, by definition, such a policy would expose the UK to periods in which its nuclear forces are inactive - vulnerable to a potential pre-emptive strike and incapable of responding.
The duration and frequency of these periods of vulnerable inactivity vary between the four postures outlined in the review. A 'focussed' deterrent would maintain a continuous patrol for a specific period of time in an environment of heightened tension or conflict, with periods of inactivity defined only by necessary recuperation. In the absence of such an environment, the UK's nuclear forces could move to a lower level of readiness such as a 'sustained' deterrent (which would always have one submarine 'on duty' either patrolling or preparing to patrol), or a 'responsive' deterrent (in which submarines would only patrol in sporadic, irregular periods). In a purely benign environment, the force could even adopt a 'preserved' posture in which submarines only deploy to carry out non-nuclear tasks and to retain the ability to move to one of the more alert postures outlined above.
The 'political confidence' a party has in such a flexible approach to nuclear posture depends upon a fundamental assessment of the threats the UK might face in the future.
The Trident Alternatives Review assumes that while the UK faces no heightened tension or active conflict, inactive nuclear submarines are highly unlikely to be the target of a no-notice surprise attack. While the reviews admits that this may not be the case during a period of hostility, there will be a period of escalation to hostilities in which the UK will be able to reduce the vulnerability of its nuclear forces by moving them from a lower-level posture (such as a 'sustained' or 'responsive' deterrent) to a higher-level posture (such as a 'focussed' deterrent). With this in mind, confidence in this non-continuous approach rests on the following questions.
First, once the UK has moved its forces to a high-readiness 'focussed' level, can it keep them there through the duration of a crisis with a small fleet of submarines? An answer to this question depends upon speculation about the nature and length of potential future crises, and the reliability of replacement submarines - something that cannot be known at this time, or (in the latter case) estimated without access to classified information. While the review states that a three-boat fleet 'would risk multiple unplanned breaks in continuous covert patrolling', it cannot clarify how frequent or protracted such breaks might be: the design of the successor submarine has not yet been finalised.
A second question follows on from this: could a break in continuous covert patrolling during a period of crisis prompt a pre-emptive strike against inactive forces? Here the term covert patrolling is important. The UK's nuclear force currently needs several days notice to fire, suggesting that it already experiences periods in which it cannot fire. However, these periods do not reveal the presence of a submarine, making it vulnerable to attack. If this covert presence is suddenly and unexpectedly made overt in a crisis, a submarine may become an attractive (and potentially irresistible) target of opportunity to a potential foe.
However, a brief break in covert patrolling may not present a foe with much of a target. Submarines patrol in vast areas, and an exposed submarine may be very hard to find, and even harder to credibly strike before any unexpected faults are remedied and the submarine returns to covert patrolling. Furthermore, the risks of an exposed submarine becoming the victim to an opportunistic strike will be tempered by the threat of a retaliatory strike by forces allied to the UK. As long as the UK remains allied to the US, such retaliation will always weigh in the minds of a potential aggressor. While it is important to question whether the UK can rely on the threat of such retaliation to deter an attack, it is also important to remember that twenty-five states within NATO (along with Japan and South Korea) are happy to do so.
Nevertheless, in a period of crisis, the risk of a pre-emptive attack on inactive UK forces cannot be ruled out entirely, and the threat (or realisation) of an unexpected and unavoidable interruption to patrolling in a crisis might pressure UK decision-makers into positions or reactions they might not normally adopt.
A third question relates to the character of the threats the UK might face. If a threatening adversary were to emerge, how would it react to the flexibility inherent in the approach to non-continuous patrolling outlined in the review? As the review states, while an increase in patrolling might cause an adversary to back down in a crisis, it might also pressure an adversary into increasing its threats - therefore escalating (rather than de-escalating) a crisis. Furthermore, the threat of such inadvertent escalation might discourage the UK from bringing its forces up to a 'focussed' period of continuous patrolling, therefore potentially encouraging an adversary further. Without a clear picture of how a future adversary would interpret a change in posture, this question is again open to speculation.
While prominent approaches to international relations suggest that an adversary would judge the UK's intentions primarily on changes to its military capabilities, a recent study of past crises suggests that an adversary is more likely to judge intention not on military capabilities, but on a subjective interpretation of a number of signals, including personal relationships and ideologies. A change in the UK's nuclear posture may therefore not be the decisive factor in the escalation or de-escalation of a crisis.
There are no entirely objective answers to these questions, and the confidence held in a non-continuous deterrent may ultimately depend more upon gut feeling than speculations about future threats. In this case, the balance between the risks of a non-continuous posture and the financial rewards offered by a smaller fleet of submarines may play an important role in determining the future of the UK's nuclear forces up to 2016 and beyond. The Conservative party has already drawn upon this to argue that abandoning permanent patrols would be a 'huge gamble' for a 'tiny saving'.
The review provides a considerable amount of information that allows readers to make their own assessment as to whether the savings are significant. It estimates that a 3 boat option would save £4 billion in procurement and support costs over the period 2016-2060 when compared with a 4 boat option, when measured at constant 2012 prices. These savings would fall almost entirely during the decade from 2025 to 2035, when savings would be around £350-400 million a year. Assuming that equipment spending continues to grow at 1% in real terms, this saving would be equivalent to around 5-6% of the projected equipment procurement budget in the late 2020's.
For project evaluation purposes, an additional annual discount on spending of 3.5% per year is often applied, to reflect the preference for spending in the future rather than now. Using this 'Net Present Value' approach, the total saving from a 3 boat option falls to only £1.7 billion. This is the figure that is most often cited by Defence Secretary Philip Hammond. The Liberal Democrats, by contrast, prefer to use the £4 billion figure, which is more relevant for comparisons across the defence budget. Both figures are right.
Both estimates appear to assume that few savings can be made from adopting the 3 boat option before 2025, for example from retiring one or more of the Vanguard boats earlier than expected, or from postponing the start of work on the first new submarine. If such savings were possible, total savings would be higher than estimated in the Report.
 Louise Edge, In the Firing Line: An Investigation into the Hidden Cost of the SuperCarrier Project and Replacing Trident, Greenpeace, September 2009
 Keren Yarhi-Milo, 'In the Eye of the Beholder: How Leaders and Intelligence Communities Assess the Intentions of Adversaries', International Security, Vol.38, No.1 (MIT:Harvard) pp.7-51
 I am grateful to my colleague Professor Malcolm Chalmers for assistance on this analysis of the budgetary aspects of the Review.